@MikeC Well, as in all things varnish...it depends. I'll give some reluctant information. This process is DANGEROUS.
When you cook resin and oil together you form a mixture. When turpentine is added to cook you alter all the materials to form a compound.
The longer you cook the resin and oil together, the shorter the open time.
Brushing viscosity is a function of the brush and added (room temperature) turpentine.
Please be safe.
on we go,
Joe