Matt Jackson wrote:

From transportforlondon.custhelp.com

"Is there a time limit for using Oyster to pay as you go?

No, if you use Oyster to pay as you go, it does not have an expiry
date. It will sit on your card until you are ready to use it again.

There is a potential problem with the Student Oysters (and others) - from
the 2005/2006 academic year TfL switched from a photo card and a separate
Oyster (which could be transferred to the new year's photocard easily) to
issuing the annual ticket on an Oyster with the photo printed on it. Whilst
the Oyster can still be used after the end of the year (with the discount
deactivated) it doesn't appear to be possible to transfer money from the old
card to the new. As one who uses the travelcard almost exclusively and
doesn't go beyond Moor Park *that* often (and almost never travelling
exclusively up there), I have very limited options for using up the credit
on the old cards. And because of the photo, I can't even give the old card
to someone else to use.