About 30 minutes. When I have mine removed at my local salon, they fill a bowl with very hot (nearly boiling) water, then fill a smaller bowl with the cold acetone and put that in the bowl of hot water. Then they put my nails in the acetone. The hot water heats up the acetone and helps acelerate the removal process. You can tell when they are ready to remove as the acrylic has gone to a mush on the nail and can be just wiped off with a tissue. Any residue can be gently filed off with a fine nail file. Your own nails will be very weak and brittle underneath for a couple of weeks until they recover. You can help this buy getting some cuticle oil and putting it on your nails to give them some moisture, don't pay for expensive stuff like Solar Oil, the cheaper oil is just as good. Hope this has been of some use.
Cheers Sue