I have read that early spring is better. Put it down even before the plants emerge. You start killing the slugs that are hatching at that time and prevent them from laying even more eggs.
Pam

Keanaz,
I would... Every one that you eliminate now gets rid of that many more eggs that will hatch next year. I would just use it in more heavily infested areas. "A stich in time saves nine".

Denny

i would use the cheap 10% vinegar or ammonia now..

and save the expensive stuff for next summer ...

ken