tldr; Scan for at least 5 seconds when looking for a beacon.
I've been playing with the ble module as a switch that scans for a beacon I carry on my keys. It works really well for turning on and off the lights in my office. The tessel is running off a battery so I want to scan for short bursts and then wait a bit before scanning again. I found that when the time between starting an stopping scanning is too short I sometimes have the relays flicker on and off. In order to stabilize the lights I've found that 5 seconds is a good time for a scan. Anything less and I get a flicker once in a while.