How often are you reading tweets of the people you follow?

Twitter is probably one of the top 3 social sharing web sites. There are millions of tweets posted everyday. Followers numbers are counted and cherished. Most market gurus use twitter for their marketing campaign.
I wonder how often people actually read other people's tweets?
Let's do the math. If you follow 425 people on Twitter and each one of them posts a single tweet everyday = you will need at least 4 hours a day to follow the links and see what is all about.
I doubt anyone will have so much spare time to do that. Therefore the Question - How much time do you spend to check tweets of the people you are interested in? On a daily basis ...