[JoshBB]

I've never done something like this but I know people who have. The basic rule is to make sure you only do it 2 days a week maximum (and spread your days out), and try to stick to around 1500 calories tops on days where you aren't doing it.

By logic you should be 3000 calories deficit from your fast days and around 300-500 per other day (so around 1500)

A pound of weight is 3500 calories so you'd lose at least a pound and a quarter a week sticking to this, possibly more depending on your metabolism.

You dont want to end up going on a crash diet where you give up and eat worse afterwards, gaining weight instead of losing it.