Maybe the max of what's already there?
Why would you add an expiration time to a UID?
I should run some more experiments, but it's unclear to me when a PGP key expires. When the first UID packet says so, or when the last? I wouldn't want to extend the expiration date of a key when adding the keybase identity. There's a note about this in the RFC, but it says, unfortunately, specifics left to implementor.
the UIDs don't expire. Only the keys do. The UIDs are valid as long as the master key is valid or the UIDs aren't revoked