The half life of caffeine being 6 hours means that in 6 hours 50 mg of caffeine will be down to 25 mg.  That’s a long time to be hyped!

Here’s a more specific example.  Let’s say, hypothetically, there’s 138 mg of caffeine in a 5 hour energy.

138
69                  6 hours
34.5              12 hours
17.25             18 hours
8.625           24 hours
4.3125          30 hours
2.15625        36 hours
1.078125      42 hours
0.5390625   48 hours to get to under a mg of caffeine

So, it takes a while to completely clear out of your system.  So much for that 3 PM rule!