At last=in the end, after much delay: I have found you out at last.
At least at the lowest estimate; if not more: This will cost me at least fifty rupecs.
I may not help you with money, but I may at least help you with advice.
At length=(i) after along time : He was in doubt for a week, but at length came to a definite decision ;
(ii) fully: He has discussed the subject at length in his article.