When you buy a mobile pass, you have to start it within 11 months of purchase, then you can use it for as long as the pass is valid (1 month for a 1 month pass, 15 days for a 15 days pass etc).

After this, if you haven’t used the pass, and it’s within 6 months after expiry, you can refund it (subject to the refund fee - usually 15%).

https://www.eurail.com/en/help/already-bought-pass/can-i-return-my-pass-for-a-refund

You can’t travel beyond the expiry date of the pass.

You must end your last journey no later than 23.59 CET on the last day of the validity of your pass. The night train rule does not apply for the last day of the validity of the pass.