Try this :
1 gel 5 min before start
1 gel every 30-45min while running

Also, you should drink at least 7oz(~200ml) of WATER with each gel. (no juice nor gatorade)
You can drink more water if needed, but it's pretty cold these days, so you should be able to drink no more than 800-900ml per hour, depending on you weight, shape, ...

Personnaly, I only take 1 gel before start with water, but that's because I'm used to it. I just don't feel that I need another gel for a half-marathon.

1/2 mary you should be able to get by on Gatorade alone--no gels, no other food, assuming you will be running < 2 hours.

Key is to have your glycogen reserves topped off in the morning of, so eat your normal good amount of carbs the day before, and get your race-day breakfast in 2-3 hours before the start (about 400 calories should do it).

Then, once you start the race, you sip 2-4 oz. G-ade at each aid station, starting right away at Mile 1. Depending on your pace, and assuming aid station every mile, this will give you 12-32 oz. per hour (1 oz. G-ade=6.5 calories), or up to 200 kcal/hour.

Even better if they have Coke or you bring your own, as it will have more concentrated calories.

But for a 1/2 mary effort, 200 calories/hour should be plenty, assuming you are topped off in the morning. And running is easiest when your body doesn't have to digest solids. But you can carry a gel, "just in case." Heck, as long as your glycogen is topped off in the morning and you hydrated well the day before and morning off (don't forget your pre-race hydration--I'd want about 32 oz. in me, or should I say to have gone through me), you can really get by on very little in a 1/2 mary. Marathon is different story, but try this out in training, of course.

Better to run a little hungry. Makes me run faster!

YMMV

I think anything over an hour is good for practice. You have to know long before race day what your stomach can handle volume wise and what will or will not make you sick. This will be what works for you.
Yesterday I had a 22 mile 3:50 training run at a slow run pace.
I had one gel before, one at 1:30,one at 2:20. A bottle of gatorade and frequent hits off the water fountains in the park where I ran.
If I had run faster I probably would have needed more since your caloric burn increases with increased intensity.
Again...find what works for you since everyones burn rate is different due to differences in body size and type and fitness level.