How will the quilt be used? If it's a wall hanging, then I would make the helixes all run the same way. If it's going to be used, like a throw etc, then letting them run in both directions will make it so there's no right side up. Which ever edge you grab, half the pattern will be right!
Barb in Louisiana's calculations are correct, however, the assumption in the calculations is that you are going to use the entire piece of fabric.
To provide a good estimate, we need to know how wide you are going to cut your strips. I don't know if the tutorial link provided by QuiltnNan is the one you are going to use, but if you cut 3" strips, then your strata is going to measure 10", and so you would cut 10" squares. The width of fabric is normally assumed to be 40", so you will get 4 squares with no waste.
If you cut your strips 3.5", your squares will be cut at 12", you will only get 3 squares per strata and lose 4".