Do you mean the dynamic address list entry created from the entry with the specified domain? Normally the TTL of the domain is used, the dynamic items are updated when the TTL expires. The doc doesn't mention that, but you can see in the recent changelog of 7.21beta:
That the TTL is used as timeout value for the entries.
7.21 only updates the behavior when you have for example both entries sub1.example.com and sub2.example.com in the list MY_LIST and both entries resolve to the same address. The entry with that address will now have the timeout of the longer TTL of the two subdomains.