How long is safe/healthy?

I'm a stay at home mom of 3 boys, I've never tried to fast before and have been struggling with weight gain no matter what I tried for the past 6 months due to my Mirena which I am finally free of.

I am going to Water Fast but want to know if anyone who is experienced/knowledgable about fasting could share with me how long I need to fast for it to be effective and safe. I don't want to just lose water weight that will return when I start eating again, I want what I lose to be permanent and big kick start to my continued weight loss.

Any help or suggestions would be very appreciated! Thanks!