Yea your metabolism will slow down during a fast, but it's a good thing if you're doing this for cleansing as well, because if you lose weight too fast you won't have enough fuel to thrive on during your cleansing! That's why I'm doing this..
It is my understanding that your "re-feeding program" or your re-introduction to food after you've finished your fast should last half the length of your fast. So if you're fasting 15 days, I'd re-introduce food for 7.5 days approximately. Of course start with juice, then raw foods, then some plain things like bread/rice, then dairy and then meat. This allows your metabolism to catch up and get back to normal. And most importantly, EXCERCISE! This will rev your metabolism back to where it originally was.
I honestly have read so much on fasting, and everyone has different opinions on weight gain/metabolism after fasting. It is easy to gain weight back and then some IF you don't eat correctly afterwards, but that goes for any diet. A fast makes you want to eat healthfully anyway, cuz your taste buds are renewed, and your body is refreshed. I dunno about you, but I always crave a nice big fat juicy apple, and even some baby carrots!
I have also read that since your bosy is renwed, it does not need as many calories to sustain it as it did before, because it is more "efficiant". I am not sure if this is true. But, it doesn't make much sense to me, because if you do cardio several times a week, and lift weights, your metabolism is gonna be revved up, and your body is going to need a lot of calories. Everything evens out, so I don't know about the slower metabolism forever, thing.
I hope this helps, and don't give up, there's others doing the same thing you are.