It may be somewhat different frequency by regional temperatures and weather etc. If the fluid in the master cylinder is not nice and clean it is contaminated by something. Rubber seals deteriorating for example.

The brake fluid should be periodically flushed and replaced. In the overall picture it is much cheaper than not doing it. The stuff will always find a way to absorb moisture . Even if it has to get by seals.

Some vehicles obviously more than others. Even condensation may play a role. In my area every three years even is far better than never. Even a simple gravity bleed is much better than nothing.

No getting the air out in most cases by a gravity bleed as well. You cannot ever get all the old fluid totally out with a twin piston caliper. Unless you take everything apart anyways.