Maybe the best answer is "As often as is convenient unless special circumstances require otherwise."
Are you running into critical issues such as storage space being chewed up by temporary objects?
If you run for a month and you don't see more than 15% or so of your overall space being returned after an IPL, and you never reach as high as, say, 85% system ASP in use, then once a month is probably more often than necessary. You might be fine running six months or more.
But if you see disk thresholds being exceeded during every cycle and IPLs are how you fix that, then maybe you should consider a weekly IPL cycle for the short term and some DASD expansion for longer term.
I have a couple small systems that I IPL essentially every night. I not only IPL, I have them physically power down and stay down for at least a couple hours overnight. I want to ensure that they both have time to cool to near room temperature before power is applied again. I want to know in the shortest possible time whether any component is operating that close to tolerance on those systems.
OTOH, I IPLed a system last Thursday night for the first time since January. It's a newer, bigger system on a much better maintenance contract. It might be three to six months before it's IPLed the next time. No clutter accumulates to a great enough amount that it's worth it more often than that.
An IPL involves multiple elements.
The system must go down through restricted state and back up again. One effect of that is that system jobs are all ended and restarted. That causes all of their joblogs to be flushed and restarted, releasing whatever memory was being used and reinitializing various things. Those jobs may run a little more efficiently until they collect temporary clutter again. Any resulting spooled files may be examined or archived or whatever. They might be totally deleted, freeing up whatever resources are associated with them.
System diagnostic functions may be executed only after power is restored for some elements. Some PTFs can require an IPL in order to replace sections of Licensed Internal Code or operating system code that isn't accessible at any other time.
There is a huge range of sites. Some need very detailed control; others may do nothing for a year or more. It takes experience with an individual system to get a feeling for it.
For added comment, you might copy the text from a DSPSYSSTS screen in the 'Discuss' area below. Also provide a general description of your recent IPL practices. It might be possible to give some more specific guidelines. Or it might lead to requests for more info in order to narrow things down.