there are different types. Some have a soak away/overflow that means with normal use they do not need emptying ( We had to have ours done once in 5 year period, when we had 3 friends staying, so there were lots of extra showers/baths and washing )
My friends had the other kind which retains everything and it needed emtying only when full (Obviously )........the timing depends of course on how much waste you are producing and the size of the tank
Kind of correct - those that have no outlet & need emptying when they're full are cess tanks, and are simply a storage facility. What I suspect you may have is a septic tank, which is more than just storage - it is a treatment facilty, using the cold anaerobic digestion process to treat the sewage and render the liquid portion 'safe' to discharge to groundwater, usually via a soakaway. A byproduct of the digestion process is sludge, and this will build up until the tank is full of sludge (and hence no room for the treated liquor). At this point, the tank will require emptying, although a little of the sludge should always be retained to 'start' the treatment process off again as the tank begins to refill (although folk will tell you that flinging a dead rabbit in does the job just as well).
How long it can go between emptying will depend very much upon the size of the tank, and the number of people it serves. Ideally it should be around 2 years, but if its a large tank and there's only a couple of you, it might last several years more. Whether or not 'grey water' e.g. bath waste & the like are connected, will make little difference, as comparatively clean water such as this will pass through the process with little effect on the sludge make - sludge is only produced by the breakdown of organic waste, so length of time between emptying is more dependant upon the amount of faecal matter, food waste, etc than volume (so don't use a food macerator when you've got a septic tank).
To determine whether it needs emptying, you really need to look at what's coming out - there's normally a kind of separator or 'interceptor' arrangement on the outlet, that decants off the treated liquor from above the sludge in the bottom but below the floating matter / fat & grease on the top. If this liquor is heavily laden with sludge solids then the tank probably needs emptying, and failure to do so will quickly result in fouling up / blockage of the soakaway, which will then result in unsightly flooding / nasty smells / a possible visit from the Environment Agency and a bigger bill to reinstate the soakaway than it would have cost to empty the tank.
Hope this helps