3 days are good... It depends in which season u r coming.
Prefer tocomein rainy season (june to aug), youcan enjoy alot at tym. Don't come in summer... April to june.
You can visit all the good places in mumbai in 2 days and you can go for some good trek in lonavala aur near by khandala also.
Take 1-2 days extra, you will enjoy inhere.... Its cheap and awesome.