Short answer: as long as you like!
Because borrowers repay monthly, your money isn't tied up for a fixed duration. If you have automatic reinvesting on, then that money isput towards buying new loans. If you don't, it goes into holding and waits for you to decide what to do with it: reinvest it at Zopa, or withdraw it to spend on whatever you like.
And if you need to access your money while it's still lent out, you can always sell your loans to other investors (rememebr there's a 1% fee associated with this).
So there's no fixed term, and no upper time limit on investing. It's entirely up to you.