what is the time complexity for binary division by repeated subtraction?
If you calculate Z = floor (X / Y) by repeated subtraction, then you will perform Z subtractions. If you divide exactly n bits by exactly m bits, then there are at least $1/2 \cdot 2^{n-m}$ and at most $2 \cdot 2^{n-m}$ subtractions, so it's $\theta (2^{n-m})$ subtractions. 

If you divide at most 2 bits by at most m bits, then Y might be 1 and in the worst case there are almost $2^n$ subtractions, so $O(2^n)$ subtractions.