First, IPL when you think it’s convenient. If you have no reason to, don’t do it.
Second, IPLs may add stress to physical parts. This can conceivably shorten service life.
Third, some hardware problems may be uncovered or exposed during IPL. IPLs are usually parts of planned outages. It might be better to learn about such potential problems during a planned outage rather than at some unplanned and potentially very inconvenient time.
Fourth, if the system is actually powered down and left down, say over a weekend, it’s not drawing anywhere close to the power it uses while running. That might be more than enough of a difference for many sites. Even air conditioning might relax a little.
There are differences between IPLs and simply bringing the system down to a “restricted state” and bringing it back up again. For many general cleanup purposes, it’s just as good to go to restricted state and back up as it is to do an IPL. Jobs are ended and restarted fresh in both cases. (For many functions, even restricted state isn’t needed. Just ending server jobs and subsystems and restarting them without going all the way to restricted state is good enough.)
I don’t think anyone can tell you how often an IPL should be done on your system. Too much depends on how your general management of the system handles all other aspects of that system.
I’ve seen every night, every week, every month, once every six months and only when required for maintenance of hardware and PTFs. I’d say start with either every week or every month, then change to longer or shorter periods based on your experience.