I dunno as soon as I notice a drop in interest, my interest drops too. If she wants to fade away then let her fade man. I stop thinking about her like a friend and more like a sexual object. I start being interested in the other women in my life, going out with my friends more. I don't sweat it, I try and stay busy.
I have a policy in my life where I never quit but always try to make things worth while for me. You never know what it's going to turn into in the end anyways so why quit? For example if I have a ****ty job, rather than just quitting i'll start changing how I work to make it worth while for me. Whatever I can get away with basically. I'll take longer lunches, try less, eat shiz on the internet more. If I have a good job I play straight ball. Same with the ladies if she stops making me happy i'll stop taking her out, i'll stop calling her as much, I won't pay attention to her, start giving my friends more priority, stop going down on her before sex, stop making sure she reaches orgasm, less kissing and romance more straight fvcking.
I start doing this as soon as I notice any drop in IL. Nothing a girl hates more than breaking up with someone who really just doesnt give a dam and is having a great time with their life. Not only that but after she leaves shes not thinking "oh yeah i dumped that poor guy" or "i devastated him" or whatever other BS they talk about while sipping mimosas. Shes won't even be sure who's breaking up with who if your quick enough.
It increases the chances of her coming back, then you can make her earn your affections again.... if you ever really let her that is. This can be alot of fun. You have to make the best of everything in life man...