JupiterBomber, I'm gunna be speaking from experience more than the knowledge of the game here (but not excluding it).
I think you're placing to much emphasis on keeping the relationship a challenge for her. I agree, to prevent a stale relationship the dynamics have to be just that - dynamic - but dont keep her guessing when it comes to actually admitting you love a girl.
I like to remember the first time a girl told me she loved me (pre-community btw) and that feeling that she made me endure (happiness, relief etc) then imagine thats how she'll feel when you tell her. Smile when you say it, sincerely, not jokingly, it adds to the feeling that your being honest.
Addressing the issue of
how many times? how often? etc. Dont throw it in once a day for the sake of it. Do it when something nice has happened; you've just finished a romantic meal she cooked, its her birthday, its your birthday (if you recieved a good present

). Refrain from using it after sex, it comes across as fake. That is, unless your laid in bed sharing an [hour long] moment.
Hope this helps
GrifterUK