Lifespan of B-type subdwarfs According to the Extrasolar Planets Encyclopedia, the B-subdwarf Kepler-70 lost its helium envelope and became a blue-white subdwarf roughly 18.4 million years ago.

(sources: archived encyclopedia page, also the paper "A compact system of small planets around a former red giant star")

This has got me curious regarding the lifespan of these stars - 18.4 million years isn't that long in stellar terms! 

Although I realise this will vary depending on e.g. mass of the star, is anything known about the length of time a B-subdwarf will remain in the subdwarf state before finally becoming a white dwarf? The information quoted above for Kepler-70 was a lot easier to find than for any other blue-white subdwarves.
In section 3 of this paper on the possible origin of two planets orbiting a B-type subdwarf, called KIC 05807616, and of which I asked a question a while ago, the survivability of the planets to the intense UV radiation of their host star is examined. Here is an excerpt from the summary:


  In section 3 we examined the survivability of the planets to evaporation by the UV
  radiation of the EHB star. Equation (3) for the evaporation rate implies that the inner
  planet will be completely evaporated within ∼ 10⁷ yr. This is shorter than the ∼ 10⁸ yr life duration on the HB.


KIC 05807616 is still on the horizontal branch (HB), like any other star of its kind, and will remain in that stage for approximately one hundred million years, according to the paper, until it finally becomes a white dwarf. I guess that's the average lifetime of the subdwarf B stars.

EDIT: I also found this paper, which is more specific with respect to the average lifetime of the subdwarf B stars, I quote from its introduction:


  An sdB stays on the EHB for roughly 10⁸ years and directly evolves along with
  the white dwarf (WD) cooling track after its core helium has
  been exhausted.