Do the tokens disappear automatically after a while?

No, but it is advisable to dispose of them annually when there are a lot of them (either this year or next) if they are not intended for export or for keeping.

There are generally no limits in IRIS, whether for groups, tokens, imports, mailings, etc. However, a spring ‘clear-out’ is recommended; for example, you could delete tokens that have already been used once a year.