It might seem vague, but every plant is different, so it all depends on the make and model of your tank as they all work slightly differently.
Maintenance and servicing is vital for keeping things ticking over nicely. Without regular care, these are some of the problems you face:
Raw, untreated sewage raising up from your tank and spilling into local streams or ditches
Toilets, sinks and showers backing up or taking a long time to drain
Overflowing toilets or sinks
Full inspection chambers (these should usually be empty)
Boggy grass around the tank
If you look after your plant in line with the manufacturers recommendations and don't put anything down your toilets or sinks that you shouldn't, your sewage treatment plant should work effectively for a long time.
Signs that your sewage treatment plant isn't working includes:
No noise coming from the plant or loud / unfamiliar noises
A white, filamentous growth over the treatment zone
Boggy areas around the tank
Floating sludge in the humus tank
No crust in the primary tank
The issues above could be caused by a number of factors and should be investigated by an engineer as soon as possible.