When using a computer you should try and get up and take a break from the computer every 30 to 40 minutes and walk around, go outside if possible etc.
If you can't get a break from the computer that often then try to get up from the computer every 1 to 2 hours and take a good 20 to 30 minute break.
I work from home and use the computer for 30 minutes to an hour and then get up and take a break for another 30 minutes to an hour.
Or if I have no work to do then I take a 2 to 3 hour or longer break and go do something else in the house or go outside and do something.
People who work in an office may not be able to take a break as often but when you can get a break you should or at least try to get up and stand up and stretch for a few minutes every 30 minutes or so even if you don't leave your desk.
I love working from home and make money blogging because I don't have to work all the time and I make money even when I don't work.
I can also take breaks as often as I want too and don't have a boss to answer too.
Sitting too long can cause health problems such as weight gain, blood clots, loss of circulation of your blood, diabetes etc.
Sitting too much is just as bad or even worse for your health than smoking.