No idea on the yield, it can vary massively depending on how good/bad a grower you are. When you say the plant is 3.5 feet tall, how wide is it? What sort of shape is it growing into? I have seen Sativas that grew straight up then started to branch out like a christmas tree, and some that naturally grow 10-20 tops and can be 3-4 feet wide, obviously the latter type tend to yield more.
Are you aware that if the plant is 3.5 feet tall when you change the light cycle, chances are it will grow to 7-10.5' by the time it is ready for harvesting? Sativas need a lot of headroom, if you grow one indoors you should flower when it reaches 1/3rd of the total height you have available, or do some extensive training!