Lifetime, in theory. In practice, you can move the relationship off Upwork after 2 years.
See https://support.upwork.com/hc/en-us/articles/360043210654-Mo...
See https://support.upwork.com/hc/en-us/articles/360043210654-Mo...
This sounds like something a pimp or a mob boss would say.
If you want to convert sooner, it's a percentage of the income for that specific client/freelancer relationship, and not total work for the freelancer. So it is probably a more palatable amount, assuming the freelancer has multiple clients.