In college I used to drink a 26oz/750ml bottle of liquor just to get "prepared" for any sort of social activity. Completely blacked out once (my memory started coming back to me in some random bathroom! And I think my liver is messed up permanently, I believe I get drunk much much quicker these days.
Oh wait, I see your question now! lol
Silly me. You need to make sure that the ONLY active ingredient is dextromethorphan. ~300mg is my very dosage, but experience users go to about 1,000mg and beyond. It all depends on the concentration and size of the bottle though. Keep in mind though, it's not a social thing, it's something you definitely do on your own (a trusted sitter is always best but I personally don't have anybody I could trust to do that.) There's an FAQ on erowid that is extremely comprehensive. I particularly recommend reading about the different "tiers" of effects - at different dosages, it starts to work on different receptors and so provides you with incredibly different experience.
For me, I prefer the robotussin gel caps. It's not disgusting to take, and 10-20 pills (half to a whole bottle) gives a decent trip.