For the best shutdown prevention it is best to start it day one, but some will run it the last few weeks to stimulate LH, plump the nuts back up, and transition into PCT easier
it just depends on how long you are running your cycle... you should run hcgenerate ES for 8 weeks on your cycle... if its a much longer cycle then you obviously want to start it a bit later... if its an 8 week cycle, start it day 1...