Chris,
90% of affiliates make under 100 a month (I think that's the stat anyway). So I would not count on making a full-time income unless you can really learn and get super good at the following.
1) How to build a good site that converts
2)
SEO - you need TONS of traffic since only maybe 1 out of 200 will buy and your commission per sale may only be around $10.
SEO help your site get top ranking in the search engines so more people can find you. There may be over 1 million pages competing for a search term relating to a product you are trying to sell so you have to be really good.
Thousands of people already know how to do this, so you have lots of catch up to do.
3) Then there is lots of trial and error in picking the right merchants and learning the best ways to promote them.
I'm not trying to be negative, just realistic. Too many people think AM is get rich quick. They throw up a couple banners and wonder why they never make a single sale.
HOW LONG? Depends partly on how much money you need to make. My best guess (trying to be realistic not pessimistic) is that if you were EXTREMLY fast at learning this and EXTRMELY good and EXTREMLY lucky you may hit 2000 in 2 years. That's based on the fact that you are starting at 0 and based on the fact that if you have another full time job you won't be able to devote much time to working AM.
If you decide to try - the biggest challenge will be the 1st few months - checking your stats and seeing 0 or $5 here and $5 there and wondering if all the work was worth it. But once you get some sales rolling, if you stay committed and work hard the commissions should very gradually continue to grow.