In my opinion 5 minute breaks are appropiate in online games. But a bigger break like 25 minutes or 30 minutes after 4 hours would be a good thing, when it is to expect, that the MTT will last around 8 hours or more. Latest after 6 hours should be a bigger break.
In live games there are no 5 minute breaks to the hour, but there are 20-30 minute breaks after 3-4 hours and this makes really sense to me, because in an other job you also have usually after 4 and latest 6 hours a break of minimum 30 minutes.
And you are free to do 5 minutes every hour, but with running time.