The leaves on Hoya hypolasia are soft and very pubescent and can be up to six inches in length.  After they have been on your plant, at full length, for 3-6 months, you are pretty much safe from having them fall off.  However, I have seen time and time again that new leaves that are not yet fully mature and hardened off can drop from the plant daily when something is not right with the plant.  I believe combinations of it being slightly too cold, or most likely the watering is not quite right will cause this condition.  The leaf will fall off within 3 days of the leaf petiole becoming slightly red in coloration.  Red buds also mean dead buds, but will take much longer to fall off.