In the AdWords API perspective, I can confirm that you can set your CrmBasedUserList object's
MemberShipLifeSpan to "10000" which should translate to "no expiration". As for the AdWords UI, I looked further into the documentation that you sent and came across this
guide, which under the "Membership duration" section, says that its duration is already unlimited by default.
However, since the AdWords UI part is already out of our team's scope, you may confirm this information with the product experts at the
AdWords Community Forum as they are better equipped to provide you with the information you need.