How long is it appropriate to keep old feature branches in git?

I am on a large team with many developers. We have a large codebase that has many feature branches from our large group of developers. We are accumulating many feature branches.

We generally look to git-flow for best practices.

How long should feature branches last?

Is it safe enough and not costly or degrading to performance to keep all of them?

Are there considerations and maintenance commands that developers need to execute every so often to have working, efficient, and responsive git ?

Thanks.

Answer

Not really git, but I've had lots of trouble with Jenkins falling over because of too many feature branches. Your question provokes quite subjective answers, I think.