If you throw yourself at it without any prep it won't be good for you.

But a proper marathon training plan at my sort of level includes a long run between 13 and 20 miles most weeks. My full recovery time after hammering for a PB at half distance is over a week but going even a little easier brings that down considerably. So a half every ~three weeks, with a couple of longer breaks here or there, should be perfectly fine. I'm not particularly planning to gun for a PB on any of these.

Doing a full is a different beast requiring more R&R afterwards, even if taking it easy, as is a fairly technical trail run, and if I'm travelling further to a new place I want time to see some of that place beyond running through part of it, hence the plan is now "several years" to spread the annual leave & travel costs out a bit more, and to allow more time for R&R and reconditioning as needed.

This way some of the more local road halfs fold nicely into a conditioning (or condition maintaining) plan for the longer/harder runs too. The longer time also helps with arranging the runs: due to the way the organised events tend to bunch around spring & autumn that becomes a constraint if you want to do it in a short time (not being able to do that particular B because it is too close to the C & D I've already set sights on, etc.)

Depends on who you are and how much you run. If you're running 50 miles a week, a slow marathon here and there would probably not hurt you too much. I know a guy who used to run a marathon every weekend through the whole season, but I think he ran way more than 50 a week.