I rarely delete old emails. Usually, I just set up rules to sort them into appropriate folders instead. But, every once and awhile I go big. Today, I deleted about 30,000+ old emails from folders that I hadn't looked at in weeks (and in some cases months). It took maybe five minutes. Way more efficient than doing it the slow way as I go.
I also set up several rules to delete incoming emails that I never end up reading. This took maybe ten minutes, but will save lots of time going forward.