It depends on how well you maintain your grips, how often you play or practice, the environment you store your clubs ( A hot garage or car trunk will shorten the life of your grips fast), grip pressure in the swing (Too tight can produce wear marks), and also the brand and type of grip makes a huge difference in the longevity of your grips.
It is possible to go many years without changing your grips. The longest I went was 6 years. I actually wore out the shafts before I wore out the grips!!! This was due to regular cleaning, proper storage, and light grip pressure. For the record, the brand and model were Lamkin Crossline Full Cord grips. I have never experienced another grip that lasts as long as that one did.
Keep an eye out for wear marks from your thumbs, feel if the grip is getting hard, slick, or cracking, and if those conditions exist, change your grips.