The standard advice is "no caffeine after 2 pm." That advice is for slow metabolisers. If you are a normal metaboliser, you can have caffeine later without ruining your sleep.
The half-life
Caffeine has a half-life of about 5 hours. After 10 hours, roughly 75% has cleared. The question is how fast you clear it, not when you take it.
The metaboliser test
Drink a double espresso at 3 pm. If you can fall asleep at 11 pm, you are a fast metaboliser. If you cannot, you are a slow one. Most people are normal.
The practical cutoff
For normal metabolisers, the cutoff is about 8 hours before bed. For slow metabolisers, it is about 12 hours. That means a 10 pm bedtime supports caffeine at noon, not 2 pm.
Caffeine after lunch is not the problem. Caffeine after dinner is.
