I need them for runs longer than an hour, which is also the general recommendation. I like Clif Shot Blocks with 3x the normal salt, my preference is the margarita flavor. I am a very salty sweater, as in I come home caked in salt after runs. I need the extra salt and the carbs and electrolytes in the bloks too. I take two after 20 minutes then 1 every 20 minutes thereafter. I also like the honey stinger chews but I only use those in the cold when I don't sweat as much, because they don't have the extra salt.
Some people are fine with just sports drinks for the half distance. It is really individual. I'd definitely echo
@Anisum that you want to try them ahead of time. They can be strong in taste and/or texture and you have to find out what works for you.