It really depends on how much space you need, what angle you're shooting from, and how tall your model is. Generally speaking I'd suggest twice the model's height for the length of the backdrop, but you may want longer.
To calculate it you can always fall back on a tape measure and some simple math- if you know how high the backdrop will be (generally assume as tall as you can go within the space available) and the distance you want the model from the backdrop (plus an extra couple feet so the edge either isn't in frame or is easy to photoshop out), you can calculate the floor length and add it to the height to get the total backdrop length you need (A²+B²=C², so in this case C²-A²=B² where A=height, B=floor length, and C=distance from the top of the backdrop to the point you want it to extend to)