For a range of instruments Interactive Brokers provide a demo account, which is 15 minute delayed, but you can use it unlimited. However your setups are wiped overnight.
They also have a paper trading account, which has identical characteristics to the real account, but you cannot open it until you have funded the main account.
However many of the brokers who advertise demo accounts won't expire them at the quoted period, because it is in their interests to keep your attention. The only thing is that they might have restrictions on the data supplied e.g. markets, instruments, time delay.
Charlton