well 3 hours is more then enough to paint 20X20 size fresco.

Painting longer than that on 1-2 sq. feet is not a right thing to do, technically it will destroy the "skin" and cause other complications.

as of burlap ones - you forgot 3 hours is the time I have to wait before the plaster is ready to paint. On ceramic tile, I start painting within 10 minutes.

in reality the only support that will give you 8-12 hours of real open time is a wall that is at least 10X10 feet.

you can upload picture here - jpeg under 150 kbytes should worjk just fine