Depends on your route. Didi from St Regis to Huanggang checkpoint should be ten minutes in the car but could be up to 30 depending on the traffic if it rains. Chinese immigration could be a few minutes or a 30-minute queue. RMB100 minibus from Huanggang (no man's land) to airport can be 30 minutes or 60 minutes depending on the queue into HK.

If you want two clear hours at HKG then I would leave the hotel at 7am to be safe. And I mean getting into the car at 7am.

If you choose different route with, for example, a single car that takes you over the border then you'd save a few minutes potentially but I'd still leave at 7am because of all the variables.