One can and should spend however much time they need. There is neither a limit, nor a minimal amount of time suitable for demo trading. If you need to trade on a demo and you don't feel confident enough to move onto a live account, by all means, keep trading on a demo account.
If someone is not confident enough then it is better to stick to demo in my opinion. And stay with demo until that person has a strategy with which he can make profit. Moving to a live account may bring loss.