Careful, there are two limits. The first, obvious hard limit as pointed out by YogurtEmperor.
The second is a soft limit denoted by GL_MAX_ELEMENTS_VERTICES. You can draw more than this limit, but an implementation is not required to perform at maximum possible speed (and some will not).
Note that this limit is not documented for glDrawArrays, but it is well-documented for glDrawRangeElements, which performs an almost identical operation (with indices in addition to a range). Both functions must lock a range of vertices to draw them, and the amount of vertices the driver can lock is obviously finite. It is therefore reasonable to assume that the same limit applies to both draw calls.
The only thing that I have seen is that GL_MAX_ELEMENTS_VERTICES applies to glDrawElements and glDrawRangeElements (and perhaps glMultiDrawElements).
glDrawArrays doesn't have a limit. It is possible that the driver does some yo-yo and juggling behind the scene that causes performance loss if you send too many vertices but there is nothing in the GL specification about limits.