The brightening can last as short as two days or as long as 1000.
Longer durations suggest either large or very slow lensing objects.
David Bennett of Notre Dame reported at the AAS meeting on two such long-duration events in which the mass of the lens was calculated to be roughly 6 solar masses, too heavy to be a neutron star and more likely to be a black hole. Bennett speculates that the lone-wolf black holes form from supernova collapse and might be as common as neutron stars in the galaxy.