How to increase the time old emails are in the trash

Mail > Settings… > Accounts > Mailbox Behaviors. One Month is the longest time available.

I thought that would be a good idea.

"You might check your email provider in case it has its own setting"

😎

I can't say. I manually empty the Trash on average, daily. You might check your email provider in case it has its own setting.

Or, wait for other replies.

My email provider dictates how long. In my case it is 1 week. It use to be 2 weeks but recently changed.