before the engine blows up, I mean 
I would think avtur will dissolve in avgas – most things do dissolve in avgas.
Would say 1 litre, or 10 litres, in a 200 litre tank be too much?
Presumably the initial hazard is detonation, but that should show up as excessive CHTs.
The worst scenario would be if the avtur is not mixed but settles on the bottom…
With a mixture of even a small amount of Jet A it is my understanding the octane will be drastically reduced and the engine will only run at full power for around a minute before catastrophic failure. You can’t detect the mixture in the 100LL by the color test. The test that works reliably is to pour a small amount of sampled fuel onto a piece of white paper. Avgas will evaporate and not leave a residue. A mixture with Jet A will leave an oily spot. If it were me and I was aware of any amount of Jet A accidentally put in the tank, I would have the tank drained completely, flushed with some Avgas, and then refilled, all at the expense of the FBO.
Ii would try that mixture in my lawnmower first 
On pprune, there is currently a thread running on this topic.
…all at the expense of the FBO.
Here in Europe, there are no FBOs. In many places, there are self-service fuel pumps and elsewhere most refuellers will make you sign a form by which you confirm what type of fuel you want to have delivered… so no one else to be blamed but yourself.
