I've always wondered how long it took other sound designers to finish a single sound effect. Sure there are those 2 min wonders, but I know sometimes a sound effect can be so frustratingly obscure (I work in videogames) that it takes me more then a few hours to get it right. I think it usually takes me anywhere from 20-30 min on average for a sound effect.
Dont forget to factor in the recording session time if you had one.