I say it's relative.
If you're a college student with little or no responsibilities, surf away! But, you should reexamine your priorties if you're constantly surfing on Friday and Saturday nights.
If you have a career, husband, and kids. You should not be surfing any more than like an hour. Think about the children and the hubby.
If your responsibilities are somewhere in between, I say no more than 2-3 hours a day.