One suggestion is to have your ready mix supplier bring a couple of test cylinders with him and you can take samples from your first load and you last load or any combination to get an average of the mix you are receiving. Have your local lab do a 7 day break on one cylinder. If you get 75% of the specified strength you're good to go. If not break the second cylinder in 14 days. If you don't achieve the 75% mark then I would not pour the floor for 21 days. If you don not achieve the rated strength in 21 days you are not getting what you paid for.

Another alternative is to pour a 5000 psi mix. You can pour the floor in 3 days with a 5000 mix. Thats what we do on commercial work when we are setting precast planking. The justification is whether an additional 15.00 per yard is worth the time savings for another 2-3 weeks wait. Depends upon what your money cost you. I'm sure there are other alternatives but this is what we do? Good Luck

P2A
Did you mean first floor walls? or first floor?

Dave

I mean first floor walls. The floor will be standard wood truses.

 No , We don't need to be at 90% strength to build the wall.

The floor will take me a day or 2 and and I would stack formers and pour off the wood floor deck on the 3rd day safely. 
 
I hope that helps.

What are you using to attach your floor ledger system? Are you familar with simpson strong-tie icf brackets. ICFVL and ICFVL-w. Depending on the size and budget you have. I work with a product  called Amdeck (by LINK) look under products tab and look at complete install guide. very slick..you can span 18-20 w/o shoring.