what is the time complexity for binary division by repeated subtraction? The divisor and dividend are of length n and m bits respectively.

According to Wikipedia article,
https://en.wikipedia.org/wiki/Output-sensitive_algorithm
division by substraction is an output sensitive algorithm and has the time complexity of Θ(Q). Can the time complexity be calculated in terms of length of input bits?
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.