How often should I flush my car engine and oil tank?

Quite often the workers at Mr. Lube asked me to flush the engine or oil (tank), how often should I do that? I always suspect those guys try to sell me the services. Any suggestions?How often should I flush my car engine and oil tank?
and that is all they are trying to do is pad their bill .unless they show you sludge from the engine ,your fine with just your oil change every 3.000 miles or three months.How often should I flush my car engine and oil tank?
Once a month just to keep it running smoothly.
if you change your oil the oil tank has been flushed


Those guys are just tiring to get some cash from your pocket