Tentacles nearest to the prey will take about 15 to close around, another ½ hour for the rest to close toward it. By that time the leaf will began to flex. Usually in an hour the leaf around the prey will fold over it & it will totally cover it by 4 or 5 hours. But if the prey is still giving a fight, the whole leaf will wrap around it like a taco in about 8 hours. A gnat, some tentacles will wrap around it but usually its dead by then & the leaf will simply bend over it somewhat if not completely if the gnat is big enough.

When you see vids of sundews enclosing prey rapidly, it is time lapse shots.

But there are sundews that do move quickly, one on top of the list is D. burmanii, easy plant to grow but is an anual. Pygmy sundews will close around their prey quickly, I've drop fruitflies & gnats, & watch in amazement the tiny tentacle fold over it surprisingly fast. Some of the easiest pymgys are D. occidentalis, D. nitidula & nitidula's hybrid. Another one I've seen flex their tentacles quickly D. binata but the leaf remain rigid & don't flex.

Oh....

Does it also depend on the health of the plant? My healthiest sundew caught a gnat yesterday afternoon, the leaf has taco-wrapped itself around the gnat and it's beginning to slowly open up again. On the other hand my weakest-which I think is also younger since it has less leaves and dew- is just starting its digestion process on an ant caught a couple of days back.