It needs to at least "look" dry before you apply any sealer. If it's still wet the sealer is not going to soak in as well and you are doing a dis-service to your client.
this has become such a pissing contest debate. I wait 24 hours, coming back the next day after cleaning. Grout or stone should be 100% dry for it to be sealed effectively in my opinion, some will disagree.
Scott, not to sound like a smart-ass but, read the label of the product you are going to use. I have seen some say the grout can be wet and others say it is best to wait 1-2 hours after cleaning.
Solvent based is the way to go less problems and better smelling. With
water based sometimes you get efforesonce in the grout lines. I've used
a bunch and found cobb's to be as good as any and better than most. Also it's
priced right and smell's good. I usually do fairly large jobs with air movers
can usually seal after about 4 hrs.