Theoretically, your supposed to replace them every oil change. In reality, they are a PITA to change, and in my case, leak after you do. I had to cut mine in 2 pieces with a Dremel to get it off. Then, after I put the new one on and torqued the drain bolt to the recommended 192 in.lbs (16 ft lbs), it leaked. After loosening and re-torquing the drain bolt a few times (making the crush washer like the one that took me an hour to cut off), it quit leaking.
Have never changed it again. Light smear of clear RTV on the bolt, proper torque, no more drips.