What does a “design life”; figure on a battery actually mean?

Design life is a classification derived from accelerated float testing at a reference temperature of 20–25 °C, not a warranty, and not a prediction of how long the battery will last at your site. The industry classification comes from the EUROBAT guide for valve-regulated stationary batteries:

EUROBAT class	Design life	Valen ranges
Standard Commercial	3–5 years	,
General Purpose	6–9 years	,
Long Life	10–12 years	ENVIROX-CEL, TOPIN (12 years)
Very Long Life	Over 12 years	ENDUROGEL, ENVIROX-CELPLUS (15 years)

Service life is a different concept, the period, established from field experience under good conditions, over which the specified capacity or power remains available. Warranty is a third thing again: a contractual term, usually far shorter than either.

The gap between them is almost entirely temperature, and the relationship is unforgiving. EUROBAT states that float operation above 20 °C reduces life expectancy by roughly 50% for every 10 °C of sustained increase. So a 15-year design-life battery at 20 °C is approximately a 7.5-year battery at 30 °C and a 3.75-year battery at 40 °C. Field observations often show an even steeper effect.

This is why a long-design-life battery can fail early in an unairconditioned roadside cabinet or a pole-top enclosure, and why the honest first question about any battery replacement is not “what design life?” but “what is the actual temperature at the terminals in February?”