Personally, I think it is a combination of the number of dives per year AND how they are distributed, both time-wise and location (site conditions) I feel there is a difference between 15 dives done in one week on a vacation trip and 15 dives scattered throughout the year at various dive sites. Doing 15 dives in a year in the tropics is a lot different from doing 15 dives a year mixed in cold and warm water conditions. I'm just using the number 15 as an example, you could plug in 10 or 100, whatever number you think appropriate.