I've not seen anything to suggest that credit on an Oyster Card disappears after any amount of time. I have one registered to me that I left at my mother's house, so she could either lend it to visitors staying with her, or use it if her concession pass got lost or damaged - it hadn't been used for longer than that and it kept its credit.
Alternatively, if you'd rather cash it in and use contactless in future, the TFL page about getting a refund for a card that's no longer wanted is
here.