If you're looking to set a personal record (PR), how often you should attempt one depends on a few factors. These include:
-How long you've been running for
-How much mileage you typically run per week
-The intensity of your training runs
-How well you recover from hard workouts
Assuming you are relatively new to running, or if you've been running for awhile but haven't been able to stick with a regular schedule, then attempting a PR once every 4-6 weeks is probably a good idea. This will give your body time to adapt to the increased mileage and intensity and will help prevent injuries. If you are able to consistently run 30+ miles per week and do quality workouts (tempo runs, hill repeats, etc.), then aiming for a PR every 2-3 months is probably appropriate. And finally, if you're an experienced runner who is regularly putting in high mileage weeks with tough workouts, then going for a PR every month or so is probably fine.
Of course, all of this is just general advice and ultimately it's up to you to listen to your body and see how it responds to different training schedules. Some people can handle more frequent PR attempts than others; it really just depends on the individual. So experiment a bit and find what works best for you!