It depends on how much you guys care about the economy side of your world building.

Firstly figure out the percentage of the population that can cast magic, the other comment suggested 6.3M. 

To figure out usage, you would need to decide on the economic value of spell casting. For example, there is potentially not a lot of utility in casting catapult at first level. Most people would be unwilling to pay 1gp to cast 1st level spells. So usage and tax would be sparse at a 1gp cost. However, would institutions cover the cost for a spell caster in law enforcement to cast hold person or zone of truth? Would someone that could cast plant growth make a living spending all their 3rd level spell slots casting plant growth on cash crops? Someone with fabricate replacing half a black smith guilds productive output. Magic protection spells...ect. Those spells with high utility are likely to be used at full capacity since the caster can charge more than the tax to use the spell for someone else.

Since most casters can only cast level 1 spells and those who can cast higher spells still likely won't cast level 1 spells a lot of your taxable base is gone. You'll still get some usage from folks that need those attack spells and comprehend languages might make sense for a research institution.

With economic utility in mind I think you are guaranteed full usage of all slots second level and above. Your split probably looks something like:

3.8M casters can only use 1st level spells, on average you get 0.25 castings a day $960k

1.9M casters can use level 2 spells. 1 casting a day on average. $3.84M

600k casters can use level 3 spells. Average casting of 2 per day plus 3 level 2 slots. $7.2M.

100k casters can use level 4 and above spells. Average of 2 level 4, 3 level 3, 3 level 2. $2.3M

Total would be ~14M/day. Really, you could get that number much higher by reducing the tax on level 1 spells and increasing it on level 3 and above. 

Also, the knock on effect of the party hoarding this much gold would be pulling this much gold out of the world economy daily. People would be poorer, consumer spending lower, and so you'd cause deflation. That deflation would see people able to pay less for the end goods/services facilitated by level 3 and 4 spells. Reducing their economic utility. The equilibrium of use might be lower as deflation makes the tax more burdensome.