I received a launch event invite on 28/5 for the event on 7/7. But I am not a sig holder and was not collecting my car on the day. My guess is that those who were collecting (just 5 I think) would have known further in advance.
I paid the day before collecting the car (which they were very polite and friendly about, but I think that was a bit last minute for them). Typically they start gently asking for payment at the point that they book in a firm delivery appointment, which for me was about 3 weeks in advance.