Once you pass the Life in the UK test you will be presented with a pass letter. This is your 'pass certificate' and you have to provide this with your future applications for Indefinite Leave to Remain or British citizenship.
Your pass certificate does not expire so you can hold on to it for as long as you like before you make your application.
However, you must not lose the pass certificate. If you lose your copy you will not be given a replacement. Instead you will have to retake the test, which includes paying the booking fee again.