The Task You Are Undertaking

You are going to be constantly reminded of how much you suck for about 1-3 months (if you learn). If you read this guide and use your brain and be actively aware of how bad you suck, you can easily shave time off of your complete noob status. You could even make some kids think that you’re really good, but only if they are bad.
Unfortunately, Dota 2 has a massive learning curve.
You need to play each of the 110 heroes at least once each to have basic understanding of the power levels of each hero.
Each game takes on average 30-50 minutes. Prepare to play a lot of DotA. If you want to cut this down, go to websites and read hero skills and guides, and from there you should get basic ideas of how to play these heroes.
You are going to have to learn items, strategies, laning combos, abbreviations, and MASSIVE AMOUNTS OF TEAMWORK.
However, I promise that once you get the basics down, and you start playing casually with your friends, that you are going to have a TON of fun.
There is no grinding for weeks to get that gear you want like in an MMO. Rolling a new hero is as easy as playing one game, and each game you can go different skill/item builds with different team setups.
Basically, it is going to take a really long time to get bored with this game. Every game is different. I’ve spent about 3+ years of my full gaming time focused on DotA and Dota 2, and I have no intention of stopping anytime soon.
You may be interested in playing Heroes of Newerth(HoN) or League of Legends(LoL), but we arguably have the best developer, Icefrog. He has been working on DotA since 2005 with no income(until getting hired by valve to make Dota 2) and no release of his real name. DotA and Dota 2 are both considered to be extremely balanced games at the professional level, and new hero and item additions always add new flavor and slight nudges to any heroes that are considered overpowered.
I see no reason to pick HoN or LoL, and please, never mention Storm of the Imperial Sanctum ever again, unless you want to try playing DotA circa 2004 with good graphics and a tiny map.