Never used plaster of paris before, but I made some posh plaster bats using proper potters' plaster they're and inch and a half thick and about 15" wide, threw a platter on one a week after I made it and at least a fortnight later still waiting for it to "pop off".
It has been damp and cold here, I've put the whole thing into boiler cupboard for occasional hour (until plaster was nice and warm) and even on top of the kiln for a bit too.
Clay is still only leather hard, about right for scraffito decorating.
In desperation I did try running a wire underneath but it's too hard for that now.
Just keep waiting?
Don't laugh if I've done it wrong!