Invites that you receive through invite holders are typically valid for 24 hours. If they expire, they get returned to the sender so they may be re-allocated - you can ask the sender to send you the invite again.
If you received your invites through OnePlus, they will be valid from 1 day up to 2 weeks, depending on the circumstances under which you received them.
You can find out exactly when your invites expire by logging into your OnePlus account. Note, however, that we are unable to reactivate an invite once it expires, as it is passed on to another user in waiting.