In a much earlier thread, i theorized that if BG had, indeed, walked the entire length of the bridge, he could have done it in under 4 minutes. The bridge is 850 or so feet long. Average man's gait is, what, three and a half feet or thereabouts? If BG was in a hurry, he could have made it in well under 4 minutes.
The latest information regarding drop off indicates they were at the trail head around 1:45-ish. According to members here, cell service is pretty good in the park and right around there (several threads back). So 20-30 minutes after they were dropped off is when the photo made it to SC.
Parking area to bridge=6-7 minutes, straight walk no stopping.
Libby and Abby take their time getting across the bridge, most folks would do the same, including myself as it appears to be a beautiful area. Maybe 10-20 minutes.
Somewhere between 2:07 and roughly 2:30 BG was on the bridge, that we do know.
If Libby's phone was "pinging" well after the murders, that indicates to me there's decent or good cell coverage at the park/bridge.