Originally Posted by
mad_rich
I wouldn't leave anything less than 3 hours. Maybe 2.5 if you have no luggage and are on a domestic flight. 3.5 should be very comfortable and will probably give you some room for a drink at St Pancras (or room for a delay)
Personally I'd risk a 2hr connection in that direction, but I know the stations well and am someone who likes to cut things fine. I think your suggestions are broadly right.
Of course if OP is arriving at T5 then flying directly to Paris from there would seem a sensible option.