how long from Kabul, Afghanistan to Hereford, England in a Gulfstream?

Answer 1

Assuming it's a G3, and depending on route, i.e. any deviations for places you don't want to fly over, you can do it in one hit ( no re-fuelling....just, it's on the limits ) about 8 hours, not allowing for adverse headwinds.

Normally the prevailing upper-level would be against you, if they were strong enough that factor might force a fuel stop, in which case you can add a couple of hours at least.