Thanks - that certainly has broadened the question as I hadn't thought of helicopters! I expect it depends on the temperature as well.
The reason for the question was that aviation fuel gets more viscosus the colder it gets, and so at low altitude it sprays nicely out of the injector, but as you go higher and it gets colder, the spray pattern slowly reduces until in the extreme condition it is just a jet rather than a fine mist. Then it is very hard to ignite.
I just wondered whether there is a measurement of how many times to try relighting, or how high or something ?