Term for timespan between generations
You mean generation time?

There are several definitions in use, but one is "the average difference in age between parent and offspring" (or "age at which members of a given cohort are expected to reproduce") which would be the same as the time period you're asking about.

(The difference between these two depends whether the population is at equilibrium or not.)

The two above are 'shortest generation time'; a third definition (which is the first in the link) is a little more complex but is more useful where individuals reproduce over a prolonged period.