Lemon oil isn't supposed to clean your fretboard, it's to keep it oiled so it doesn't dry out and crack.
I'll second that.
Also a good tip, use old t-shirts with cleaning products. They don't leave fluffs on the guitar/strings (like the fast fret rag for example). I just cut uppan old t-shirt and put a bit in each guitar case.
No more often than twice a week even for the very frequent player. Once every string change should be enough.
