Well 20 minutes would be aboout right if the gates aren't far apart and there's a transit security check (which you need to go through between the arrivals level and the departures level) in the vicinity. It could take up to 40 worst case.

For dining (assuming no airline lounge access): https://www.hongkongairport.com/en/shop-dine/dining/
Select "Restricted Area" to show only those outlets in the airside departure level.