ATPL's and time to CPL

[Gillespie]

Once you've passed all of your ATPL exams, how long do they stay valid? Ie, must you commence your CPL/IR within a given time scale or can you relax a while in order to save for the next module?

whilst I'm here, what must you do in order to keep a CPL current, (ignore the IR for the time being)?

Thanks in advance