As others have pointed out, there is "servicing," where the reg is fully disassembled and cleaned and parts that are subject to wear--mainly o-rings and valve seats--are replaced. And then there is mere "inspection," where the reg might be subjected to some tests and visual inspection to see if all is good.
Manufacturers' recommended service intervals may differ by manufacturer. The manufacturer of my regs recommends servicing every two years or 200 dives, whichever comes first, and inspecting them more frequently than that. Don't interpret this as my advice, as each of us needs to do what we're comfortable with, but I try to keep in mind that regs are regs--the basic architecture doesn't differ by manufacturer--yet some recommend annual service and others two years. My conclusion is that manufacturers don't tend to determine their recommended service intervals with great scientific precision.
And then there are the people who choose to go much longer intervals without servicing, though I would like to believe they are inspecting their regs often.