My engineer looks mine over approx every 2 months-it is however working for about 5 hours every day Mon to Fri.
Ther ecan be a variety of reasons why I nned to cal him out-loose cable, on/off switch starting to go, small leaks, heater not working.
Each time he comes out he'll usually look over the vacs and pump to see how they are performing and advise accordingly if they are getting near to needing to be replaced.
Having built up this wroing relationship its got to the stage where if say a vac goes in the afternoon, he'll call of an evening to put the new one in ready forthe next day, so realistically I have no down time.
Obviously i pay more than many for servicing costs but as I don't lose out on non-cleaning time I think overall I'm in profit..
Rog