On Account Funds - Credit Expiration?

We currently are offering an on account credit instead of refunds for those who can't come on their original date/time to our museum. We want to avoid having a huge "on account" balance and would like to put some guidelines of an expiration date that would convert the on account credit to a donation.

For anyone currently doing utilizing on account funds:

• Do you "expire" these funds and if so, what is your expiration timeframe? (1 year? 2 years?) We've had a suggestion for 6 months, but it just feels too short with COVID.
• When the funds expire, are you converting to donations? If so, do you send a receipt acknowledging the donation?
• How are you communicating the expiration date? On your website, in email correspondence?
• We were thinking about trying to set up an HTML email confirmation type that explains the on account credit and expiration so it could be triggered when we put funds on account in a transaction. Not sure if this is possible--anyone done anything like this?

Would love to hear how anyone is set up for success in keeping control of their on account funds or any tips for managing the conversion to expire them!