Alex,

This is a great question and very pertinent in today's time of computer, and video games being common and a part of most children's lives.  I think your question can be answered in two parts.  Firstly, the limits set would probably relate to the child's age and status in school.  For example, if you have a 17 year old junior in high school who is an excellent student and able to balance both school, and excessive video game play, there probably wouldn't be any reason to limit his game playing for any other reason that you may want him to spend time with you.  On the other hand, if you have a 10 year old who is struggling academically and or socially, and loves excessive video game playing, you may want to set some guidelines such as, 1.  Video games AFTER homework, or 2.  Video games if and only if you maintain grades, and lastly, 3.  Video games after some exercise and or social interaction.

Every family and parenting style is different with focuses on different things.  Some families are against excessive television, which would then include video games.  I'm including some reference articles for your perusal.  

http://www.education.com/reference/article/Ref_Video_Games_Cons/

The second article answers more of your question regarding limits
http://www.education.com/reference/article/Ref_Parents_Media/?page=2

http://www.education.com/reference/article/Ref_Television_Family_2/

http://www.education.com/reference/article/Ref_Healthy_Habits_TV/?cid=70.100

I hope this helps,
Best,
Rachna