Amt of time for my BMR to go up after fast?

Im on day 2 of 40 on water cleansing. Im doing it to cleanse my body bite me. Before, i maintained at about 1,100- 1,300 that suxx >:( anyway, i was curious if you guys know if thats going to decline my metabolism drastically? I heard water cleansing/fasting can make it go down up to 75% ! I know i'll gain 10 from water weight- and im going to eat slightly more to up my metabolism (with the right foods) but my question(s) are

How much lower do you think my maitenance will drop (700/ 800 cals a day)
How long will it take to up my metabolism after the fast (and consume slightly above maitainance to get my BMR higher) a month, two?

Thank you so confused >.<