How often should I re balance my 401K?

There is absolutely no set rule to this.

Each person has their own criteria for their portfolio. Some have more risk tolerance than others.

Some people feel the need to monitor their investments closely and others don't.

Some people are invested in individual stocks - and in general those require far more evaluation than mutual funds. Others are invested solely in one target retirement date mutual fund. Some people might feel this requires zero monitoring.

You are the one who determines how you want to be invested. You are the one who has to decide if you have a strict asset allocation you want to maintain or if you want to let your winners run. You are the one who has to make sure your portfolio is invested the way you want it to be. That might mean you look at it weekly, monthly, quarterly, yearly or once a darn decade. It's up to YOU.

Once every six months or once a year is plenty, assuming you have an appropriate Asset Allocation to begin with...

it's up to you. One way to rebalance is to simply change your contribution allocations; sometimes it's best to do nothing.