First, sounds like you did all the right things as far as curing is concerned.  Did you also put expansion joints every 8-10 feet (or so)?  This is critical for crack control...

Concrete will reach about 70% of its design strength in 7 days, reach or exceed design strength at 28 days, and keep gaining anywhere from 10% to 100% during the next couple of years.  These are "rules of thumb" and every concrete is different.

I assume the slab was not 'designed' so there is no 'design strength' to reach.  Normally, it would be safe to drive and park after the 28 days.  A lot depends on the base under the slab.  It is carrying all the load and if it "gives' even a little the slab can crack.  If it is well compacted then you can probably drive on it after 7 days.

One thing to look for is 'edge curling'.  When concrete dries faster on the surface, the corners 'curl' up.  This may not be noticeable to the naked eye, but as you drive over these corners that are no longer supported by the ground they will break.  I doubt that this could be the case with your curing procedures, but I would stay off the corners for as long as possible.

Hope this helped...