Is it obligatory to inform the fiancée about practicing masturbation for 7 years, out of fear of wronging her, and is recovery possible after quitting it, and how long does that take?

You are not required to tell your fiancée about the forbidden act of masturbation you fell into. Rather, what is obligatory is to repent to Allah Almighty by abandoning the sin, regretting it, and resolving not to return to it. Allah accepts sincere repentance.