How much longer can I use my S21 Ultra for?

As long as you'd like, it won't stop working because there's a new device out.

It also has another 4 years of security updates, if that's what you're trying to ask.
 
Samsung is offering 3 major Android OS updates & 4 years of security updates on the s21 line.

I wouldn't see the need to retire it anytime soon unless you really want a new phone.

Edit: My post got 'stuck' in a loop & meanwhile, the above responses posted. Now my post is rhetorical.
:)
 
I have an essential phone from 2017 that capped out @ android 10. I installed lineage os on it and it serves perfectly fine as a backup phone. So to sum up, if it works, as long as you like.
 
Should I trade mine in?
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