Coincidentally, we had our septic tank emptied a couple of days ago.
Turns out it didn't need emptying but we bought the house 4 yrs ago and had no idea when it was last emptied and what might have been put into it. (Or even what sort of a septic tank we had!) So we paid €165 for someone with a truck to come and suck the solids out and tell us if it was looking OK.
He said it all looked fine & was nowhere near full, but good idea to get it emptied from time to time. (Well he would say that I suppose?)
All his comments were basically very reassuring ie tank is fine and large enough for the house size, very deeply buried and having a good hill assisted soak-away area. So probably no need to empty again for 3 or more yrs. Maybe never I'm thinking now, as we are quite careful with it and only 2 of us in a largish house? (It might just rot down inside on its own and never get to a dangerously full state?)
He filtered the water part of what he sucked out and put that back in as the water contains good microbes? (Don't need to waste any of those!)
I think he said that when you look inside, (if you know where the opening hole for emptying it is) if you see a crust, that means it needs emptying. (Does that sound correct?) Otherwise I guess one finds out there's a problem if/when the shit comes out of the loo instead of going down to the tank?!
In brief, as far as I can tell, how often you need to empty a septic tank depends on a lot of factors eg
the type of tank
size of tank versus number of ppl using it
what goes down into it (ie shd be only bio waste and degradable loo paper - nothing else and limited or no use of bleach)
the degree of digestion achieved (we put a dead mouse down sometimes as apparently that's good, others put a tablet in once a week or whenever)
its physical location (ie on the flat or down a steep hill like ours)
We still don't know how many litres ours holds. (We forgot to ask.) The chap reckoned we don't have a grease trap. He also couldn't find our ventilation pipes. And there was a mystery valve just outside the house which he didn't understand ie what it was there for. (And of course neither did we!) So the septic tank remains somewhat of an enigma, but as long as it works, no need to worry?
Can't ever completely forget that we have a septic tank tho, as there's the occasional whiff of "drains" in the air either outside or inside one of the showers - just to remind us! Ah well, it's the countryside here. (Told to run water down sinks and showers is there's a smell as the water trap has probably evaporated.)