My indoor cats have rabies vaccines every year, its required by law here, but also because there is the possibility of rabid bats or mice getting indoors.
We have decided (my vet and I), based on the latest research, to give the other vaccinations every other year, as they are not at risk of contacting other cats outdoors on a regular basis. I would worry if they were never vaccinated, because there is always the chance of contact if they run outdoors. She says that the newest vaccines last more than one or two years, and they don't really know how long, because the research studies were only designed to test efficacy for one year. (they did not set out to test, how long does this last?, but rather, does this last for one year.)