Depends what you wish to achieve.
Do you like classical and want to be able to play classical songs? Then go for normal piano classes. Let's just say your target is Grade 8 and each Grade took 1 year (example). Do the math. Depends how fast you pickup and practice, you might take longer...
Also, make sure you're into classical songs.

If you just want to be able to play contemporary songs, there're many types of courses available.
Try:
http://www.playbyear.com.my
You'll be learning how to play music by ear using the piano/keyboard without any notes. The cost is considered very cheap if compared against classical piano and it's much much faster to learn, it's utilizing modern teaching methods.