As long as you keep it clean, you rarely have to condition the leather. I have a customer who has a 1996 Mercedes E320 that I wax about every 3 months, but I've never detailed the interior-no real need to. The guy is a neat freak and I know for a fact (because he said he's never put anything on the leather) his leather care consists of wiping it down with a damp towel when he washes his car. His leather is still soft, supple and clean.
If the leather stays clean, once a year is sufficient.
