i've acquired a couple of rental properties now and during every inspection they always suggest replacing the electrical panel, because it's old.    They of course fine, but how often do you guys replace them?  just for the sake of replacing them.

We often replace fuse boxes, even older breaker panels, but never just for the sake of replacing them.

If by "old" do you mean a fuse box, then it is also quite probably your wiring is "old" as well ... most probably ungrounded and possibly even knob-and-tube (KnT).  In those instances we would incorporate a {partial} rewire into our renovation plans.

It is also possible you have a breaker panel from a defunct manufacturer for which you cannot easily secure new breakers.   There are some breaker panels which have a history of malfunction and instances of fire ... your insurance provider would likely encourage you to replace them.

Finally, if you are upgrading/modernizing the electrical in the building, the existing panel(s) may have insufficient capacity: you may need/want to upgrade the entrance size and/or may need to add more circuits than the panel supports.

No, I don't replace for the sake of replacing. Unless the box is super old like the fuse-style, it's likely staying. The inspector may just simply just be mentioning stuff for your information and to beef up his report to you.

We replaced the one in our personal home not because it was super old, but because we were adding a lot of of breakers and needed a bigger box and setup for more power.

At our primary residence we replaced it because it was really old and my husband was worried about the possibility of a fire. I have not done that for a rental.