Been doing a lot of online collaboration lately.
With particularly tough problems, it's easy to start to burn out even even after only 70 to 90 minutes of work...
...so we noticed that, and started implementing 10 minute breaks. I've got a couple music playlists that go 10 minutes exactly, I put one on, and when it ends get back into work.
It's working really well. If you find yourself grinding at the computer, try it out. If you're collaborating via Skype, definitely try it out. It's been a boon to productivity and quality of life.