Now to start this off, the chances that you are having too much caffeine through something like coffee is highly unlikely. The possible overuse of caffeine is going to come largely from powdered energy supplements, mainly straight caffeine powder. Although there are some side effects of caffeine that will affect nearly everybody, the likelihood of death by caffeine is small. Now, there are certain side effects of caffeine that might happen due to a slightly too high caffeine consumption.
How much it takes to kill you: A lot
According to USA Today, it would take about 50-100 cups of coffee to result in a lethal dose. The large variability is caused by how much a person weighs and how well they metabolize caffeine. Much like alcohol, the more you weigh, the less it affects you, and some people just metabolize it faster. Now, pure powdered caffeine would result in a much lower amount needed, but this is often only sold to commercial, scientific, or business buyers only. Largely due to the fact it can be lethal if too much is used at once. As far as pre-workout goes, it will take more than 1 or 2 scoops to create any ill-effects and much more than that to be lethal.
How much to cause some unwanted side effects: A decent amount
About 5-8 cups of coffee, resulting in over 500 mg of caffeine. This can cause some jitteriness, nausea, headaches, and accelerated heart rate. Although none of these are likely to be lethal, they are unwanted side effects that many people would like to avoid. This is still more than the average amount, and I can generally take in 500-800 mg/day before I start to feel this.
How much should you have: Enough
If you average about 3-5 cups of coffee per day, then you will likely be fine and not be anywhere near the ill-effects. I will often have coffee in the morning then my pre-workout in the mid-afternoon to make sure I have the optimal workout. Find out how much is too much for you, then go ahead and stick around that number. Sometimes you’ll overdo it and you’ll know it. If a cup of coffee makes you bounce off the walls, then you likely don’t need a pre-workout with 350mg of caffeine. Don’t have too much, be smart, and continue to get after it.