Cathay opens check-in 48 hours prior, so you can check in via app and get your boarding pass provided they don't need to check your documentation at the check-in gate.
I'm sure you can get away with getting there 6-8 hours early... but the question is why go that early?

I would normally chill around IFC and grab a drink on the rooftop or near Central before taking the Airport Express, and since you only have a carry-on then it's not too cumbersome