I do my own servicing and expect to get at least 100 dives and at least 2 years of use between major services (for me that means I have to replace an HP or LP seat). I'm probably getting 3 to 4 years and more with just some minor tweeking. But the biggest variable is what you do in terms of cleaning, storing, and inspecting your gear. You need an IP gauge to check your reg properly and detemine its health. I have had serviced/new regs sit on a shelf (dark, climate controlled & no high O2 source) for 3+ years and check out just fine and perform flawlessly.
On the other hand, if you don't do a good job maintaining your gear, an annual service may not be enough.
Less than competent annual service can cause more problems than it fixes or prevents.