Hiya,
if you're talking about Water Fasting it's usually not recommended you go beyond 40 days as this is when the body starts to eat itself and starvation begins, you could kill yourself going over this amount of time. You could try alternate fasting, so 30 days on juice and then 30 days water. I read somewhere it's not recommended you do more than 2 x 30 day Water Fasts in a year. Hope that helps!