Our New KickStarter! Espruino Pico

The bit about the 10 year runtime on an AA-sized lithium - might want
to clarify that. Most people will assume that means " Lithium", the
1.5v alkaline replacements, not the 3.6v lithium thionyl chloride cells that I assume you had in mind.

Current draw in sleep: <0.1mA - over 2.5 years on a 2500mAh battery.

Whoa ...The minimum "Pico" input is 3.5-20 vdc.

Are we talking 1 or multiple batteries?
Are you using a booster on the battery(s)?
What type of battery are you using?
Are we reducing the capacity 8% per month for self discharge? (for lipos) Varies between other battery chemistry.