If you take a strong enough SARM, your body will enter a state of suppressed testosterone due to the fact that it was regularly receiving an artificial source of testosterone from the SARMs for an extended period of time. Thus, your body forgets how to produce its own testosterone.
When you perform PCT, usually for a period of 2-4 weeks, but sometimes longer, that’s generally done to allow your hormones and all of their levels to stabilise back into something resembling normal, instead of the radical mess that SARMs leaves them in.
Returning testosterone levels to SARMs probably requires at least 4-6 weeks, assuming that the PCT is also being done and finished during those weeks. If you can’t do the PCT or something else goes wrong, it may take longer for your testosterone levels to get back to the right level.