I think they can carry the sperm for more than one lot of eggs (like guppies) and they lay when they are good and ready. I have a seperate tank that I put the females in when they are ready to lay. One female lays early and frequently and the other later and infrequently (one has laid 3 lots and the other none yet). I can tell by their behaviour when they are ready to lay. They get very restless and dig in the gravel frequently, then they dig in the gravel on the basking area and when they switch from digging with the front legs to the back it is time to move them. In the laying tank they get very restless and get up-down-up-down for a few days then lay. They are looking for the best place to lay but don't have much choice.