Is there a general formula for this given the boundary? I saw how optional stopping theorem is used to derive the expected stopping time for 1 dimension, $\mathbb{E}(T)=ab$ if walk is between $-a,b$, but when I tried to use the same method for a 2 dimensional box it doesn't work, there are more probability terms than the number of constraining equations (in 1 dimension there are 2 probability terms and 2 linear equations constraining them).
What is the method for calculating $\mathbb{E}(T)$? in 2D, 3D, and higher?