Setting up custom aam_uuid cookie is optional. The value of demdex cookie and custom aam_uuid cookie will be same that is : AAM UUID. Demdex cookie has a TTL of 180 days, please check this doc : Audience Manager Cookies
I will check with the team if we can increase the value of custom aam_uuid cookie from the code implementation or not and get back to you.
The demdex cookie which is default and is a third party domain cookie has a TTL of 180 days, since the value of demdex cookie and the aam_uuid cookie (first party domain) has the same value, so that should not exceed 180 days as well.