As others have said, Tilly, de-sludge once a year. It could easily go for longer, but, eventually, sludge would find its way into the soakaway system. That would mean a new soakaway if it couldn't be got at to clear it.
So, very common here in the shires. Nothing to be afraid of.
One big BUT though. I've installed plenty of modern ones ........ big plastic round thing. If possible, find out the age of the installation. Older ones tended to be crude brick/block chambers. Probably still OK, but should be checked to see if it's still working properly.