Well, if you want to get hardnose about it, it is technically stealing because it is the companies computer, connection, and time. That being said, most places make an allowance.

Where I work, you are allowed to use it for personal reasons with certain restrictions. No adult or hate sites, period. Your use cannot interfere with the completion of your mission. You should try to limit it to your own time, like lunch and breaks.

There are also cases where personal use can benefit the company. For instance, you have to do some banking transactions. The company would rather give you fifteen minutes to complete it online than for you to have to take an hour or more away from the office to do the same thing.

As far as "excessive" use, that is up to management to decide.