you can flower now but the yield is going to be greater for this grow if you veg longer. the choice is bigger plants maturing more slowly with higher yield vs.smaller plants maturing more quickly with lower yield. if you are planning on growing continuously, the majority opinion is that you;ll get more bud with many smaller plants
as for yield on this grow it depends on too many variables to give an accurate estimate - your experience, strain, light conditions, nutes, etc. a wild assed guess would be for a couple of ounces per female plant.