For LIFT, they can grind a lot off the base circle and you may need longer pushrods. But almost any duration can be ground.
A flat tappet cam has a taper to the lobe's nose that spins the lifter as it rubs past. Really, the whole lobe is tapered so the base circle still trys to spin the (Semi) flat slider lifter. The lifter face is also slightly convex tapered to help this spin.