A 75cl bottle of ~40% will do me good for a night with nothing else. If I'm having a few beers too, I usually have 1/3-1/2 a bottle. Most of the time I use spirits from my hipflask, so I'll use everything in my hipflask on a night out, so a 75cl bottle will last me 6-7 nights out.
It depends really, also you have to take into account that I'm rather large and a moderately heavy drinker, if you've never drunk before and you weigh 6st I reckon you could get wrecked on 3rd of a bottle.