How can I tell how long (more or less) it will take me to complete a game?

Use HowLongToBeat.com.

You can use HowLongToBeat.com. It collects stats from gamers and provides averages and so on:

How it works?

Check the column that best matches your play style, this is the median time it will take you to complete the game. You can also click on the games individually to really break down the stats.

• Main Story (Required) - Gamer completed the main objectives, just enough to see the credits roll.
• Main Story and addition quests/medals/unlockables - This gamer likes to take his time, discover and complete additional tasks not required.
• Completionist (100%) - Gamers that get every achivement, every medal and conquer all that the game has to offer.

If there are a decent number of stats for the game in question then this likely provides a more accurate picture than a review would, unless you happen to find a reviewer who plays creepily similarly to you :P

(Hat tip to Mehper C. Palavuzlar.)