At what point would it be OK to remove it from the propagator and transfer
to a windowsill?

That depends on the conditions at your windowsill. You will want to keep it
growing until the leaves yellow naturally or force it into dormancy by
witholding water. It will want a dry (but do not dessicate), frost free
(45-50F would be good) winter. Think benign neglect and check for mealies and
scale. Underpot in well drained potting mix. Avoid overhead watering. No
fertilizer until spring.