That is a subjective question that depends majorly on you; how fast you can learn, how much time you can spare, how diligent do you think you can be.
Maybe a better question would have been "what does it take for you to be able to participate in bounties", when you understand what it takes, you determine how long it will take you.

Btw, bounties is the one with higher requirements, airdrops are daily easy if you have a Facebook account with 500 friends above and/or a twitter account with 500 followers and above,