Many dive shops recommend servicing every year, while many manufacturers recommend servicing every 2 years or 100 dives. The difference is explained by the history of recreational diving equipment. In the early days of diving, the wear components (primarily the o-rings & seats) and lubricants were of a quality that most manufacturers recommended annual service. The labor cost of service and the material cost of a few soft parts was such that this was a reasonable course of action. More recently, materials have improved considerably (the industry has switched to fluorocarbon based synthetic elastomers and lubricants as a result of widespread adoption of oxygen enriched Nitrox) plus labor and material costs have skyrocketed. Today, it wouldn't be unusual for the cost of two or three overhauls to approach the cost of a new regulator.
In the past few years many regulator manufacturers have adopted the every 2 years or 100 dives recommended service period. A few manufacturers have switched to recommending an "inspection only" every other year and a complete service every 2 years or 100 dives. A very few manufacturers have not yet updated their service recommendations and still specify annual overhauls in deference to their retailers. Dive shops that have service departments often retain the very conservative annual service recommendation regardless of manufacturer policies. Some experienced divers perform a periodic inspection themselves and do not service their regulator until it exhibits signs of wear (typically an unstable first stage IP or a dribbling second stage), which means their regulators often go for many years between overhauls. Thus the answer that service frequency seems to be more a matter of opinion and experience than science and research.