Cleaning a guitar is totally dictated by use, acidity/oil/dirt/grime left by the player, and time. There's no set rule because each factor affects the other in one way or another. For instance, let's say I play my guitar for 4 hours a day, at least, I don't wash my hands before I play, I don't clean my strings when I'm done, etc. And I sweat a lot. The fingerboard won't need the moisture treatment as much as it will need to have a top layer of gunk removed from it after about 2 weeks of playing. With that level of playing, I'd treat it with lemon oil about once every six times I cleaned it.
Speaking of which, one of my reps just left and he's got a buddy that has developed his own brand of guitar polish and his own brand of lemon fingerboard cleaner. The cool part is I get to put my name and logo on the bottles.
I just ordered a dozen of each and I'll let you know how they work out. I ask a lot of questions about finish testing and it seems to safe for all finishes to this point. The developer of the formulas comes from a chemical background in paint/coatings manufacturing.