Without luggage you can make it in under 20 mins - all things going well and not arriving at a very remote A40 gate or so.
With luggage you should also be able to make it in under 1 hour. 45 mins sounds about right on average.
..Usually you have an extra 5-10 mins as the trains hardly ever run on time nowadays anymore ... except when I need to make a tight connection, that is the day they are on the minute ...