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

A very vague question. Depends on the game. RPGs and Open-world games like GTA tend to be longest, anything marketed mainly at kids/movie tie-in tat generally the shortest. Some people see 8 hours as a long game, others want 40+ hours of gameplay. You really just need to look at a forum for whatever game it is, or google it. There's no average.

The only way to accurately assume how long a game will take other than looking up reviews and specifically searching for estimated completion times would be to have first hand experience with the game (if part of a series). There's no easy way to look at a game and immediately assume it will take X hours unless you have a lot of experience gaming.

Needless to say, RPGs are the longest to complete, typically ranging between 20 and 40+ hours (with Bethesda RPGs assume longer).

First person shooters are more varied, with some lasting around an hour (Rogue Warrior) but most lasting between 5-10 hours for the single player campaigns alone (Call of Duty, Battlefield, Bioshock, etc). Assume online multiplayer can consume 100's of hours if you're dedicated.

I'd go into more detail/depth but honestly it's a combination of the game, the competency of the player, the gaming experience of the player, and the style of gameplay (achievement hunter, casual, speed run, etc).

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.)