This study[0] shows that it takes about 4 hours of complete inactivity to see a large decrease in LPL (lipoprotein lipase) in the legs. It then takes about 4 hours of low intensity activity to replenish previous levels.
Of course, then the question becomes how much low intensity activity do you need during the day to offset sitting. If you avoid sitting for 4 hours straight, do you avoid most of the bad parts? Is standing alone enough low intensity activity to keep your LPL going or do you need a treadmill desk? Could you go 3 hours sitting, 1 hour walking, repeatedly and still have good LPL levels?
The research is only clear on one thing: don't sit for 4 hours straight. It'll take 4 hours of walking around for your LPL levels to recover.
[0]: http://www.ncbi.nlm.nih.gov/pubmed/12815182
