It depends on the disease as to how long your immunity keeps up antibodies to it.
As I'm in a high risk job for tetanus it is advised to booster every 5 years to a total of 5 boosters to give lifelong cover.
There are plenty of older dogs still becoming ill with the likes of parvovirus, I personally wouldn't risk it. Then there are the dogs who manage to get to ripe old ages of 15 years plus, who's immunity is reduced naturally, if their vaccination program is kept to date they will be protected at an age where they really don't deserve to die through lack of immunity to disease

I think it is distemper which tends to last a minimum of two years immunity whereas lepto or hepatitis ( cannot remember which ) has poor immune properties so often dog a re not fully covered for the full year.
I know plenty people who don't vaccinate after the first booster but it is a game of Russian roulette in my opinion. If you are really worried about the risk of routine vaccines then at least have titres done to see if your dog still has immunity. Nursing collapsed, dying animals with diseases that could have been prevented is heartbreaking
