They have various sizes of tankers, so they'll send one out appropriate to the property - same thing with septic tank emptying, if you're considering one of those as well. The Scottish Water one that comes up here drags a second tank behind it, so you'll quite often see the second tank left in a layby while the tanker goes somewhere slightly less accessible to empty a septic tank and then hooks back up and pumps from one to the other. If it's one of the old concrete-style pits, it may never need emptying, they just look after themselves for the most part as long as you don't put too many chemicals down the drains. That said, I need to find someone with a digger to come and excavate the end of my outflow pipe because it keeps blocking and I'm getting grey water spilling over the top of the tank into the field, which is a bit niffy (but the grass is looking FANTASTIC!)