The longest you can ! It's also going to depend on how big the moon is in the sky, etc. etc. etc., so it's an impossible question to answer in a way.
Samyang do a couple of good performing 500mm lenses. Only about £100. That's prob about 150 dollars ?
It's also quite bright, I would use a spot meter and you may have to use + one or two stops. No problem with digital
You can buy telescopes and all sorts. The Tamron 150-500 if you're getting serious. A 500mm on APSC = 750mm.
Needless to say, a good heavy tripod will be very useful. Possibly a right angle viewfinder, too, to save your leg or neck lol
You can check out Samyang lenses on Ebay. Don't be fooled by their low price either. From what I've seen of their shorter line up,
the lenses are stellar performers.