Best case about 90 minutes on pretty clear roads (which is not going to happen at 4:30 on a Friday). Worst case - who knows? I wouldn't be surprised if it took 3 hours or more.
Public transport would be much more reliable for timing - DLR to Canning Town, Jubilee line to Baker Street, Bakerloo to Paddington, then Heathrow Express, shows as 1 hour 7 minutes on the TFL journey planner. Slightly longer, but a lot cheaper and less changes is DLR to Canning Town, Jubilee Line to Green Park, Piccadilly Line to Heathrow; that comes in at 1 hour 25 minutes.
http://journeyplanner.tfl.gov.uk/use...T2?language=en