What is the life expectancy of a bumble bee?

Lifespan varies depending on both the species of bumblebee and the role/gender of the bee.

Here are some typical numbers though:

For Queens, about 18 months on average - enough for them to be born, live through summer, hibernate in winter, emerge in spring and set up their own colony during spring/summer. After that they die.

For female workers, the average is about 4 weeks, but it does vary by species - for some as few as 5 days, others maybe 6 weeks. Threat-level will also affect lifespan - workers that remain in the nest during their life may well live longer (I myself have recorded a lifespan of 100 days) - those that forage may become injured or predated and thus die prematurely.

Males (Drones) only live about 2 weeks, during which time they must mate.

Note that the lifespan of the colony as a whole is a different thing. Typically a bumblebee colony will last about 10 - 14 weeks.