It took you 18 hours? 

My first playthrough took maybe 8-9 hours playtime. I finished a later playthrough in one sitting (Maybe 4 hours), having collected all little sisters, upgrades, and tonics.

I remember when I was playing bioshock for the first time I was also playing dead space, and I'd heard rumors saying that dead space was disappointingly short and that bioshock was good-size, and it ended up being the opposite.