Using hcg during a cycle keeps your testies working for the whole duration of the cycle. This is crucial for keeping the testicular function of your testies and it will make your pct much easier.
If you consistently used hcg during your cycle, using hcg after the cycle is just to make the transition to natty a bit more seamless. You can use it a week into pct, and by a week into pct I mean, a week after the aas has totally cleared. For E/C esters that is usually anywhere from 2 to 4 weeks, depending on the dosage used. For 250mg's 2 weeks tends to be enough but for 500 or above it's going to be 3 or even 4 for 750 - 1g doses.
For normal cycle durations (ie. 8 to 16 weeks) after one full week of full pct, in my experience, you should be producing LH at 50 to 100% normal capacity. This is why I would stop using hcg at this point to avoid any further suppression of your natural gonadotropins. If it was a super suppressive cycle, nandrolones plus more then two compounds (the more compounds you use, the harder it is to recover), then imo two weeks of hcg isn't a bad idea.
You should also note that hcg on cycle doesn't work the same with all the different compounds and dosages you can take. You might need more or less of it for heavier or lighter cycles, to produce the same results. For trt test dosages it has been researched, that 250iu e48h, produced almost 100% of intratesticular test compared to baseline. So with light cycles I would use 250 and with heavier 500 e48h. But if your hcg is half bunk, then you might need a bit more.