how frequently should numbers refresh on an exercise machine?

I'm building an application that runs on a tablet connected to an exercise machine (a rowing machine)> The tablet is running a video of the instructor, and overlaying the video is the performance of the rower - current pace, number of meters completed, stuff like that.

Some of these performance stats update very quickly - the datastream is 100Hz. It doesn't make sense to update the numbers that often. They'd be a blur. On the other hand, updating the progress every 2 seconds is frustrating. As a rower I am spending all my attention "waiting" for another update when I'd be happier if I was "free" to focus on other things.

I looked around for some literature about perception on updating numbers and I couldn't find any thing relevant. Anyone have a pointer for me? How often should the numbers update on the tablet?