6 to 7 hours

the biological haft live of Magnesium in human body

Due to its metabolism, a living being can get rid of some of the isotope's atoms before they decay.

Half-Life, Half-Life: Blueshift, Half-life: Opposing Force, Half-life: High Definition pack (might not classify as a game), Half-Life: Source, Half-Life 2, Half-Life 2: Episode 1, and Half-Life 2: Episode 2. Eight games total not including countless mods created by fans that you can download for free on moddb.com and fileplanet.com. Other games by Valve software include Left 4 Dead, Left 4 Dead 2, Portal, Portal 2, Counter Strike, Counter Strike: Source, Team Fortress, Team Fortress 2, and Alien Swarm.

Another Half-Life game would be Half-Life: Decay. You might also be able to count Half-Life Deathmatch and Half-Life 2 Deathmatch.

you start with half life 2, then you play half life 2 episode 1, then you play half life 2 episode 2, so you are basically playing them in the order they are in at the game select screen (where you choose any of the half life games, portal or tf2)