Hi there, the caffeine is very stable at all times, so there is no change by time. The best coffees are grown in high altitudes, so they use to have less than -40% of the caffeine that have other coffees grown in low altitudes.
The choice is to find a high altitude coffee to drink the best espresso.
Also - you have a nice site. But may I respectfully say... the background image of the horizontal lines is a little hard on the eyes - even though it is only visible for a few seconds.