How long does it take to proximity wingsuit fly for 100m?

One of the level 45 goals in Alto's Adventure is to proximity fly (fly very close to the ground without touching it) for 100m. The game does not record your attempts ("best is 45m" or something like that), so it's really hard to tell how close you are to achieving it.

The game might tell you your current distance on the goals menu if you manage to pause it during a proximity flight, but I've never managed to do it long enough to try. But if I knew about how long it took to fly that distance I could at least count the seconds so I'd have a ballpark estimate.

So: about how long does it take to wingsuit 100m?

Straight from the horse's mouth: it takes about 5 seconds. The developer shared a video of the achievement. The successful proximity wingsuit flight begins just after the second rock, and the achievement popup comes up after about five seconds of proximity flight.

(n.b. I still haven't achieved this, but at least now I know roughly how close I'm getting.)