But how do I set rules without sounding badly demanding?

What's wrong with being "demanding" about your needs, wants and desires? Humans have sexual needs, just like they have love/affection/intimacy needs. These are things that your SO needs to fulfill if she wants to continue a relationship with you. Just like you fulfill her needs.

If you are unsatisfied or unfulfilled, then you have a right to demand more. But do so with respect and kindness.

And what if she just forgets about the rules?

If you talk to her, discuss what you need and what she needs out of your relationship, and then come to an agreement, then yes you need to make sure she follows through.

If she "forgets", you remind her. "It's been 1 week". Personally, I am more crude and say things like "I feel full"(balls are full of cum). But my SO likes that sort of talk.

If your SO is a prude, you're better off just reminding her how many days its been and that you are feeling sexually frustrated.

What are the fuckups that i should watch for? :)

Stand your ground.

Since she has a low libido, sex is a "chore" for her. She doesn't want to do it. She'll try and get out of it. You have to stick to your position and you'll be rewarded for it.