No the vacs don't need changing every 3 months. I find they are now lasting over 12 months between changes (don't buy cheap)
The vac motors in my current everyday machine are date stamped 2003
Mind you, I did only buy it a year ago and it had been very lightly used in the first 5 years of its life I think. needs the brushes doing now though, get a nice little burning smell when I switch on
The machine it superseded used to pop a motor about every 2 years or so.
I just changed its 135psi pump, good lord the difference is stupendous!! I think I killed the last pump because I've been experimenting with ultimate master and crystal green in-tank, I think I have clear and conclusive evidence that detergent powders kill pumps. Shame, because they can really make a difference on a minging carpet
I suppose if I add a tenner on to every job that I need to use an in-tank detergent, once I've done more than 20 of them I've paid for a new pump, kind of.