Oct 19. 1970. RAF Marham. Victor K1a tanker. Eng failure on lift off. Fuel jettisoned - heavyweight circuit flown - landed 150,000lbs.
The captain was FC. landing weight indicates that 36,000 lbs fuel were dumped at circuit height 1,500 ft.
Fine drizzle was falling and the good citizens of Swaffham had a very strong smell of AVTUR fuel. A colleague of mine driving to work put out his cigarette. It was a monday and despite the drizzle the good houswives had washing out to air. Most of this had to be rewashed.
On another occasion a fighter tanking fuel broke his probe during fuel transfer. The resulting stream of fuel (immediately behind the basket) doused both his engines (subsequently relit).
As has been pointed out above however, in dry conditions the fuel vapourises quite rapidly. It also gently falls lower initially. I would say, that a good separation to aim for would be the same as is used to allow wake turbulence behind a heavy to dissapate.