There's no real rule of thumb. This is however, a good guide to go by (Time on + wait time for PCT + PCT time).
If we look at younger testosterone replacement therapy (TRT) patients that were successfully restarted, they are treated and tested 6 weeks post treatment (this is why I recommend blood work 6 weeks post PCT), and then another test in 8 weeks. At this stage a doctor can determine if the individual has recovered successfully. That's 14 weeks. I add another 4 weeks for good measure considering we are not all young, we are not all using testosterone replacement therapy (TRT) doses and our cycles are considerably heavy.
Some folks don't recover. The more time you give your body to normalize, the better your chances at increasing recovery rates for future cycles.
It's very individualistic. The chances of a full recovery is as close to ZERO as it gets. So if there is any chance at minimizing that gap, it would be the wise thing to do. Too many things are involved in the chain of recovery. If the hypothalamus recovers and begins sending Gonadotropin Releasing Hormone (GnRH), it doesn't mean the pituitary will respond. If the pituitary responds, it doesn't mean the testes will function. So you could become either secondary (pituitary) or primary (testes) hypogonadal. Because there is so much involved and it's not like a "switch" you can turn on for all chains, you need time for everything to balance.
Your HPTA is as strong as the weakest link.