In addition to (and inspired by

) what Mike wrote above, here is my analysis on the effects/benefit of the supply boost spell provided through the academy. This is entirely based on the information available in the wiki and my own lvl 15 elven workshops.
Comparing the use of a spell on a 1 day production with a 9 hour production
We're going to try to get the maximum out of this spell, so we'll boost a level 15 elven workshop just before a one day production has completed. This gives us 10150 supplies upon collection. Thereafter, we'll produce 15 minute productions until the spell depletes, of which we can make a maximum of 5495 supplies (7x 785).
The grand total is 15645 supplies and it would take us 26 hours to make. Unless you actually plan on going away for an entire day – this is actually quite inefficient. Here's why:
Let's activate the spell just before a 9 hour production completes (which is what most players likely use during a normal day (ex. while at work) or over night/ whenever one goes to sleep). Upon collection we now 'only' receive 6925 supplies. After completing a maximum of seven 15 minute productions until the spell wears off, we'll have a grand total of 12420 supplies. But hang on! Isn't that less than the first example? Yes, but not when considering that you had 15 hours prior to setting that 9 day production to produce supplies at any rate of your liking in comparison to having set a 1 day production. To stick with a simple and plausible calculation, let's assume we completed one 9h production and two 3h productions before setting that 9h production we'd later cast the spell on. The total of supplies you've now produced equals: 18390 (that's nearly 3k more supplies than the 1-day method!)
We can therefore continue to follow the strategy of shorter productions being the most lucrative – regardless of the use of spells or not. This assumes however, that you have the chance to collect finished productions and set new ones roughly at the time of their completion of course.
Comparing the use of a spell on a 9 hour plus 7x 15 minute productions with building a level 15 elven workshop instead of the academy (a little more detailed than Mike and using one level 15 workshop instead of two level 8 workshops)
As this spell takes 16h to produce before the next can begin production, we have a 16h time frame to complete supply productions in our eleven level 15 workshop before the potential 2h bonus of said spell would actually come into effect for the
first time we make this spell. Thereafter the time frame would be reduced to 14 hours (as we can use the spell while already producing the next, decreasing the time in between the use of the next spell if used consecutively). For the sake of simplicity and plausibility, let's go with a 9h, 3h and two 1h productions during this time frame of 14h for our additional workshop. This would give us a total of 6026 supplies. As we're not limited to collecting productions within exactly 2 hours without using a spell, we can actually complete eight 15 minute productions in ~2h. Using a spell, that is indeed impossible to do, even if we're just 5-10 seconds past the 2h time limit – the spell will always have worn off before we manage to click that 8th production.
Now to compare the benefit (if any) the spell gives us with just having one extra workshop instead, we need to take into consideration the following:
a) In either scenario, we have a level 15 workshop producing supplies at a 9 hour cycle, and 8x 15 minute productions thereafter (however the last 15 minute production wouldn't be boosted when using a spell).
b) We therefore have one completely additional workshop! This means that all of the supplies this workshop is producing are to be considered a bonus.
c) The bonus received through the spell is thus actually just the 150% additional supplies we collect during it's duration of 2 hours.
The numbers:
With an additional workshop we would produce a grand total of 8538 bonus supplies.
2770+1600+2*828+8*314=8538
With the use of a spell we would receive a grand total of 7452 bonus supplies.
2770*1.5+7*314*1.5=7452
The conclusion:
The additional workshop wields more supplies, without the cost of any relics (don't forget building the academy costs boosted ones!). In addition, there is currently a soft cap on how many spells can be produced and thus used, while the workshop will always produce supplies and you can theoretically even more than triple the amount of bonus supplies received through the use of shorter productions.
So why build an academy?
Despite the above, there are still two huge benefits the academy grants you that we have not taken into consideration.
1) The use of one spell can almost grant you the same number of supplies as an entire workshop, without the cost of population or culture. This could allow to upgrade buildings to higher levels in your city, or the need of one or even two less residences and build a culture building in its/their place. Thus granting you a significant increase in culture that may just grant you a higher culture boost. (One less workshop + one/two less residence/s = potential space for one academy costing 0 population and 0 culture plus 1 or more culture buildings of your choosing.)
2) You're not fixed on only increasing your supply production, but can also boost your goods production in a single manufactory without the need to tear down the workshop and build a manufactory in its stead. Plus, you can now place a temporary boost on any non-boosted manufactory. This is extremely useful should you lack a specific good due to insufficient trading partners and choose to produce it yourself instead.
A message to the devs:
The biggest down side is the soft cap and difficulty of gaining relics! Therefore relics should become easier to acquire and
certainly not be limited. I'm confident the devs have a plan to eradicate this issue through a further new feature (like that competition/event previewed), and at the very least let us know about it very soon. Just telling the community that it will be possible to permanently and continuously gain relics through other means than encounters at some point in the future would already suffice.
Thanks for taking the time to read this book of mine.
~ Valerius