Not knowing which one of resins you are pouring I would say anywhere in the ballpark from 90 seconds to a couple of minutes...I pour RC-3 into rather small molds and have that window of 90 seconds to a couple of minutes to pour
I don't know if the short open time resins that Brian is talking about will turn well on a lathe. They might, I haven't tried. I'd suggest you look at Amazing Clear Cast, a slow set epoxy with a demold time of about 6 hrs. It cuts well and polishes up nicely. The urethane resins (Clear, Clear Slow, Water Clear) also cut well and will polish to a glass-like finish, but they need a pressure pot for bubble free castings.