Being a Steam user certainly has it's downside when it comes to the annual sales. The Summer Sale is probably the one that hit me the hardest as I bought almost $100 worth of games just in the sale alone. The Fall and Winter Sale's were not as damaging but were not without roughly 10+ games bought from those as well.
1. Play the indie titles first! Ones that have just a couple achievements tied to it (less than 10, or up to 15) usually are the ones with roughly 5 hours of gameplay to them. The only exception to this rule are the rogue-likes, which can take anyone 50-100 hours to fully complete if going for all achievements and not just play from beginning to end.
2. Not sure of the average length of a game? HowLongToBeat.com is your friend! This site provides the average length of just about every game in existance based on user feedback. This site also includes the fastest completion, main storyline, and 100% completion times. An excellent source to pick out the shorter games from the long.
3. Game first, achievements after. If you're like me and you absolutely have to get all (or most) the achievements, make that your second priority when it comes to longer games. If you worry about achievements, you'll only be wasting more time on a game when you could be playing something else. A great example of this is Assassin's Creed IV: Black Flag. To play through just the main story could take you roughly 20 hours the first time through...however if you aim for all the achievements during your playthrough expect to spend a good 50+ hours...That's 50 hours you could have spent knocking off another 2 or so games in your backlog!
4. No achievements? No problem, play to enjoy!
With these points in mind I will most certainly be able to get through roughly half, maybe even most of my backlog by the end of the year, but I won't call that definite for sure. Anything can happen that may potentially turn my attention away from Steam altogether for an unknown amount of time (such as my slightly growing backlog of PS3 games...).