We do not know. It may not even be the same "point of no return" for each person.
All that we can ascertain from Scripture is that when a person
DOES reach that point, they will have no desire to repent and will be unfazed by the likelihood of going to Hell when they die. Those are both functions of the Holy Spirit Who will have left at that point. Hebrews 6:4-6 makes it clear that that condition does not apply to
- unbelievers,
- new believers or
- those with OCD;
but to mature believers who have made a deliberate, informed &
protracted decision.
"For
it is impossible for those who
- were once enlightened, and
- have tasted the heavenly gift, and
- have become partakers of the Holy Spirit, and
- have tasted the good word of God and
- [have tasted] the powers of the age to come,
if they fall away, to renew them again to repentance,
since they crucify again for themselves the Son of God,
and put
Him to an open shame." Hebrews 6:4-6 NKJV