I think for most people between 3 and 5 years is good. I think in my NY house we waited 10 and the guy said it was "time".I'm not trying to be offensive, but I would seriously question your specialist based on this statement.
You not only have solids build up from the bottom, but you will also have a scum layer forming from the top down. By the time ~50% solids build up from the bottom up, the scum layer is going to take up most if not all the other space and there will be zero room for water, which will mean a backup.
Systems can be pumped completely empty and will still contain enough beneficial bacteria on the walls and surfaces of the tank, to restart the digestion that happens in the tank.
I do not believe you can pump it too often. It's not like they pump every last drop out of your tank.