When it is really dry, about 3 heats per set. You can use them longer, but they loose that little edge that can make a difference. Schum yellow mini spikes work too, but I find minipins a bit more progressive and predictable there. Plus it is one of the controll tyre for our nationals.
I hope I see you at the GP. By the time I wanted to inscibe in january, it was already fully booked so I won't be driving.
And being a member of the local club doesn't help. It seems off road is on the move again. They never had the GP fully booked so early.