After playing for several years, I spent a few years marshalling at a couple of sites, although not concurrently.
The first one was unpaid, not even petrol money, but my two lads got to play for free; I was the pyro marshal (I have a bit of professional history with explosives), so had great fun blowing chunks out of earth mounds, old cars, ponds, the general landscape and, on one occasion, a structure (oops!). All of the marshals thought nobody was getting paid until we found out that the chief marshal was getting £250 per day. By then, my eldest son was also marshalling; we resigned at the end of a day which had started badly and got worse after a row with the site owner. The chief marshal was sacked for supporting me and another marshal resigned in solidarity.
Not long after that, the marshal who had resigned in solidarity and I started playing at another site where, after a year or two, we both got roped into marshalling on the basis that my younger son could play for free (his older brother had given up airsoft as he now had real rifles to play with) as could his son. However, we soon realised that the guy running the site was a bit of an arse and that most of the other marshals were a waste of space; the best run day was the occasion when the two of us were the only marshals on the site. We quit and went back to playing at the site.
Since then, two site owners have asked me to marshal; I have learned my lesson so politely turned them down.
Marshals are essential to airsoft; however, too many sites abuse their goodwill.