How often should you ask a client for referrals?

Never. If you have done a good job they will give them without you having to ask.

Once. If they don't refer after you ask once then they probably never will.

Every couple of months. If you don't stay top of mind they won't remember to refer, so you must remind them on a consistent basis.

Every chance you get. You can't ask too much, so every time you communicate with them you should give them a way to refer.