Very tight especially with Brexit looming. When we were there, there was a minor slowdown/work to rule at the port and it slowed down traffic into the port considerably - like hours long. When we left the port, we could see trucks backed up for absolutely miles. It was fortunately not too bad leaving the port, but if Brexit causes any problems with customs or cargo shipping, it could be significant. Given the botchup that is Brexit, I would personally be expecting a mess at Dover.
We disembarked about 9:30 and got to Heathrow a little after 12:30. We were originally booked on a 6 pm flight and got notification that morning that it had been delayed to 10 pm. There was a flight at 3 pm we had originally declined to book because we were concerned about not making it (we'd seen news articles about issues at the port which is apparently quite common). We had thought that maybe we'd try to switch to the 3 pm flight but it was sold out and by the time we finished checking in, it was 1 pm which would be cutting it close for an international flight. If we'd disembarked early, we could maybe have made it (if it hadn't been sold out) but given the "normal"-ish traffic we experienced, I wouldn't have wanted to be worrying about getting there in time if there was worse traffic.