how long before a 5k-10k even do you drink your coffee?

Coffee is mostly water and is absorbed rapidly. It does not take several hours to digest a few ounces of liquid; in fact, you could probably drink it safely during the race. My suggestion would be to drink your coffee 30-60 minutes before the race, just long enough to allow the caffeine to kick in.

I always have a coffee every morning, so race day is no difference. Before workouts i usually have a coffee two hours before hand so i try to stick to that rule of thumb. Drink it, as normal as possible so your body gets used to it. If 1 hour before hard workout, do one hour before race...

I usually have it between 1-10 months before.

I generally try to drink coffee 3 hours before a race.

A seasoned coffee drinker should not feel much of a dehydrating effect. Especially if you drink e.g. a glass of water after the cup of coffee. This dehydrating get is a bit stretched imo.

I like milk coffee with sugar, but before a race I'd refrain from adding milk for sure. A bit of sugar is fine though.

How long before? That is easy: I make good coffee at home, espresso, to be more precise. The coffee "out there" is 99.9% crap. So I drink coffee before I drive to the race! ;)

Caffeine peaks in the system 2hrs after ingestion. Take caffeine pills, it is far easier to regulate the amount of caffeine you are getting. A coffee could be anywhere from 20mg-80mg... where as with a pill or half a pill you know you are getting 50mg or 100mg.

2-3mg per kilo of body weight is proven to be helpful, anymore than this is not helpful and may be detrimental.

I find the possible gastrointestinal effects (ie getting the runs!) more of a concern than dehydration from drinking coffee. I'm a 1-2 coffees a day person usually; generally doing 2 a day sessions - most often coffees are with lunch between sessions; never before an early AM workout and rarely before a morning race, and at least 2 hours if not 3 before an afternoon workout or race.

It's wise to be conscious of drinking an extra glass or two of water per cup of coffee though, especially in warmer weather.

Long enough to poop before the race.