That seems to be a popular topic today, with two pieces in the popular press exploring that concept, albeit from very different viewpoints.
The first, from Wired writer (and Mythbusters host par excellence) Adam Savage relates the value of deadlines. He claims that he usually allows projects to languish on his workbench when given open-ended timelines, but while 'under the gun' performs at a high level and (in his words) finds solutions that are "innovative, elegant, and shockingly simple."
|Of course, there's always that third option...|
Credit: BBC | Wikipedia
Chemistry Angle: I like both approaches, but find they work best at different times in project cycles. While you're slogging through, trying to find those optimized conditions or exploring new leads, it's best to follow the '90-minute sessions' approach. Take a break! Nearly every scientific autobiography I've read suggests that problems get solved when the authors step away and do something completely unrelated for a while (Kekule, Feynman, Newton, Watson, Fleming, etc.)
Adam's approach (Hurry! Time running out!) fits best when you have a specific goal in mind - getting that paper out, presenting at the conference, filing that patent. Though I hate to admit it, the 48 hours leading up to submission often produce the bulk of such efforts. Sometimes it's worth the extra stress - and extra coffee - to get a superior final result.