The F check in people have opened check in for me 8 hours before my J flight to London; but I had no bags to check. Checking in bags may make it a bit difficult ...
It's a bit hard to give more of an answer/opinion without more information. Where are you coming from, what time is your flight and why do you wish to check that early?
I can assure you Qantas F lounge in SYD T1 is nothing to rave about.