Pure hike took me 2 hrs. Photo-taking leh, took me 3 hrs even when i u-turn halfway at ranger station.
But if we start 8.00 a.m. its unlikely we miss last light (I hope). The bridge closes at 5.00p.m. so if you not there by the time, yo ucan't get on it anyway.
There is a short cut via a lane along Thomson Road, need to bash through a small path (30 meters) to join the main track (very safe) and you can save 20-30 mins each way, i.e. 40-60 mins return trip. It took me about 1.5 hrs to reach the TT walk.