i'm mining with bitminter client with my GPU that is an "ATI Radeon Caicos PRO Prototype" with a speed of 20Mhps.. how long does it take to make 1 bitcoin mining this way

does it worth it? or how to easily get bitcoins fast for free??? i don't wanna buy bitcoins...

Basically never with a GPU....
Try an ASIC for BTC or put your card to work mining scrypt(n,jane, adaptive-n),x11,sha-3,or blake256 coins. Personally I see potential in x11.