Just to update something Gonzo said.. it's 6 months now - between completing a rating course and starting a unit training plan.. otherwise you have to do an APC (a few days back at the college to remind you/prove to instructor that you can still do it all!) before you can begin the training at a later date...and that's provided that your student licence hasn't expired! (2 years).
So yeah, you'd want to be getting valid!