Just to clarify... I'm talking 2 or 3 drops across the whole fretboard. And of course, this will vary depending on your other habits. For example, I wipe my guitar down with a dry microfiber cloth after each playing session... So end up removing all of the natural oil from my fingers (and the gunk too -- which is my purpose). But all that wiping down removes natural oils from my hands that would otherwise be there (and be beneficial to the fretboard).
This is my routine with the guitar that I am playing at that time... Guitars that are sitting in storage get oiled twice a year (but probably 7 or 8 drops across the entire ® fretboard). As long as you wipe off the excess, lemon oil will not harm your fretboard.
But Garret is absolutely right, I am probably being excessive -- I'm OCD. But a little too much oil is far better for the life of a fretboard than too little -- and easier to remedy (again, just wipe off the excess vs replacing a dry / cracked fingerboard).
Here's a good experiment... Go to a pawn shop and look at some older guitars. Look at the fretboard under the low E string around the 20th fret (the part of the fretboard that gets touched the least), then look at the fretboard under around the 5th fret. You'll quickly find a guitar where you can see a serious difference between those 2 locations on the fretboard -- you'll see the difference in the health of the fingerboard... Because most people NEVER oil their fretboards -- and that is criminal. Especially if you've taken the time to invest in a quality instrument like PRS.