EK First Class Lounge Dubai
 

I plan to fly Emirates First Class BKK-DXB-ORD; I have the option of an 8 hour layover, or 5 hour layover. The 8 hour layover would arrive DXB at 1AM, the 4 hour layover would arrive DXB at 5AM. ORD flight departs at 945am. At those odd hours, do I go with the shorter or longer connection, if I want to enjoy the F class lounge?

IMO 4hrs is more than enough to even try all 3 F lounges.
I've spent 8+ hours there before but only out of necessity due to long connections. There's not that much to do

Nearly five hours is plenty of time to enjoy the lounge.

I'm on a similar itinerary, flying EK 373 BKK-DXB and out of DXB at about the same time as your ORD flight. Yes, it's a long layover, probably too long, but since you ask in the context of wanting to "enjoy the F class lounge" it might be worth pointing out that, as far as I can tell, the menu changes at 4am from 'lunch & dinner' to 'breakfast'. So if you arrived on the later option, you would not get access to the full 'dinner' menu. Breakfast menu is nothing to complain about but there are many more choices on the dinner menu. You can compare here to decide for yourself if worth coming early for: https://c.ekstatic.net/ecl/documents...urse-a-dxb.pdf

You might willingly choose an 8 hour transit over a 5 hour transit once. You won't do it again.

Even 5 hours is stretching things as there's only so much you can do in an airport lounge.

Actually, personally I’d take an 8 hour overnight transit in F over a 5 hour one at DXB any day! But my objective would be to book a hotel room and go spend 5 hours sleeping rather than spend it in the airport - it’s much preferable if you have a passport that allows easy entry into the UAE. My limit is about 4 hours in a lounge before I start to feel like I’m trapped in a never ending loop these days. If 5-6 hour overnight layover options show I pretend they don’t exist.