I have been using diphenhydramine (the active ingredient in zzzquil) as a sleep aid off and on for over 15 years. If I use it a few nights I tend to start being reliant on it....but the reliance will go away after 1-3 nights of not using it (I can usually find a time when I can have a night or two with a little trouble getting to sleep like on the weekend). Honestly its best to just play around with it and see how it works for you. You might try taking just one pill the first time and see how that works. I got a sample of the zzquil liquidcaps to try and the packaging is VERY difficult to open so I recommend buying benedryl (or store brand diphenhydramine) instead. That's what I have been using since my teens.
In anycase-you can use it just as needed, or everynight if you want-I have done both and both worked fine. Every body is different though so like I said, try some experimenting.