if you are a healthy ADULT you can fast 15-30 days if you do it properly. however you should never set time to fast like that and you should be willing to break the fast. i did a 19 day fast last year, you can look up the log over at evo.
remember during a prolonged fast your blood pressure and adrenaline will be sky high. you are in flight/fight mode. so prolonged fasting should NOT be done by those who already have high blood pressure or if you have a tough time hydrating during the summer months.
also if your goals of fasting are injury repair you should be taking time off of the gym and getting plenty of rest during this time. if your goals are simply weight loss then you can continue working out as long as you hydrated.. many different strategies here and if you want to learn more click my Ebook in my signature or contact me for a consult on this.