As I said, in my testing I found it to be almost immediate. I closed a request with my account in one browser, then went to another browser where I was logged in as you, loaded a different request and saw that your account could vote for it.
Discourse still keeps a record of your vote for a feature request (that’s why everyone’s icon is still showing on that closed request). But that vote is no longer counted against your number of allowed active votes.
I just closed a feature request you voted for. Can you now vote for another feature request?