Robert M. Ewers in Nature:

The 34 interesting talks lasted, on average, a punctual 11 minutes and 42 seconds. The 16 boring ones dragged on for 13 minutes and 12 seconds (thereby wasting a statistically significant 1.5 min; t-test, t = 2.91, P = 0.007). For every 70 seconds that a speaker droned on, the odds that their talk had been boring doubled. For the audience, this is exciting news. Boring talks that seem interminable actually do go on for longer.

He also gives some good advice on giving a good talk:

To avoid banality, speakers should introduce their objectives early on and focus on pertinent information. They should avoid trite explanations, repetition, getting bogged down by irrelevant minutiae and passing off common knowledge as fresh insight.