If you were an employer, how much internet surfing would you tolerate?
As long as my employees' objectives are being met, and projects aren't being constrained, I probably wouldn't have a problem. If I noticed it happening far too often, then I'd just give them more work to do. The only time I surf the web at work is for work purposes, or when my tasks are completed. Though to be fair, my computer isn't in the same space as the labs, where the majority of my time is spent.