gamma ray burst, extinction-level event
As bon noted in the comments, that it the time it would take the gamma rays to reach Earth. Gamme rays travel at the same speed as all other forms of electromagnetic radiation, including visible light, so we will not have any warning. This kind of makes the "6000" number meaningless. We will know that the star exploded and the gamme rays will reach earth at the same time. The destruction of ozone will then probably be instantaneous (on a geological time scale).

From your quote:


  It has been suggested that a supernova or gamma ray burst caused the End-Ordovician extinction.


It has been suggested because there was no other plausible terrestrial cause for the mass extinction, so people started to look at extra-terrestrial causes. But a paper published earlier this year documents a mercury anomaly associated with the extinction event. This gives strong evidence for a volcanic cause for the extinction (similar to the P-T or K-Pg events), and there is no need to make up a gamma-ray burst.

David S. Jones, Anna M. Martini, David A. Fike, and Kunio Kaiho: A volcanic trigger for the Late Ordovician mass extinction? Mercury data from south China and Laurentia. Geology, July 2017, v. 45, p. 631-634, 2017, doi:10.1130/G38940.1