Oanda’s demo account doesnt expire… I can also suggest global futures, while the account does expire but it can be extended with a simple request to their customer care… But Oanda would be my first choice…
Just to clarify, an FXCM does not expire as long as you trade on it at least every 30 days. If you go 30 days without placing a trade on your FXCM demo, then it will expire. You can always register a free FXCM demo here.