depends on temp etc and it will disintergrate before it will go mouldy most of the time but i can put peice in for a day or so
course none of my fish like to eat it, but its a good way of removimg my hundreds of snails
There was a thread on this subject recently and I believe that the consensus was 24-36 hours. Cucumber and zuchinni will often look fine in the tank until you touch it, it then disintegrates into a real mess. If you think it's gone too long remove it with a net - not by hand.