there is no set time or mileage for a regen , every truck is diferent , it shouldnt make any noise during regen , just should be slightly under power when driving ad slightly higher idle if sitting still, its possible they dot know what to do to fix your problem, and may be using that excuse tohope you buy it , may want to get with the dealer and let them kow you do feel there is a problem, and also tell them , you think it does regen to often, and you wouild like to at least have it checked , i do agree with driving style and load , things like that effect the regen process