Never cuff a woman that you're having sex with casually. A vast majority of the time, women that you are FWBs with are in 2 catagories:
Type A: the most don't give a damn about your life, nor what you're going through, you're just an option in case she wants some sex. Situations like this normally work out, because you don't really care about getting rid of her like that, nor would it be that difficult to get her back.
Type B (most common):
The one that really does like you, cares about you, and wants to be with you in the end, but is willing to move at your pace. This sounds great, but a lot of these women are low-key thots. A lot of dudes can get in their ear and get them to do anything. Furthermore, a lot of them don't like you, they just like the challenge. As soon as they get with you, the feelings are going to slowly start to fade away. Challenge is gone.
Any respectable woman would leave as soon as she realizes that she is a friend with benefits. she would fall back completely unless you called her and said that you wanted to be in a relationship.