align-region-heuristic is a variable defined in `align.el'.
Its value is

If non-nil, used as a heuristic by `align-current'.
Since each alignment rule can possibly have its own set of alignment
sections (whenever `align-region-separate' is non-nil, and not a
string), this heuristic is used to determine how far before and after
point we should search in looking for a region separator. Larger
values can mean slower performance in large files, although smaller
values may cause unexpected behavior at times.

You can customize this variable.