Another telltale sign of impending doom is whether you smell any burning of an electrical nature. Usually the motor is close to becoming toast. It is usually less expensive to buy a new vacuum. If you stay with the same brand, you can use your old accessories with the new model usually.
I had a Shop Vac brand that I also used for collecting drywall dust, etc. I tore it apart and cleaned it up real good. It lasted for another two years or so. Right before it bit the dust (no pun intended) I had just bought a HEPA style filter for it.
I replaced it with another SHOP VAC brand. I kept all the accessories. So now I have 4 extension pipes that will let me reach 12 foot ceilings with a breeze. I have 16 feet or so of flexible hose. Of course that newer HEPA filter also works on this new VAC. I only had to invest another 5 bucks to get a coupler that permitted me to join the two flexible hoses together.
I also saved the power cord as sometimes these give out and I saved the canister as well as its casters. I now use that to store garden tools all together in a corner of the garage. When you want to clean - VOILA! the whole thing - tools and all moves as one easily out of the corner for simple cleaning and sweeping.
Who could ask for anything more?
A friend owns a Craftsman Shop Vac and it does have a lot of power and longevity - at least the one she owns which is probably 15 years or more. I like her shop style vac. It has incredible sucking power!