Should one repent of the sin of neglecting prayer now, with the intention of not returning to it, or should one repent at the moment of returning to the sin, or should repentance be delayed?

Repentance from all sins is immediately obligatory, and it is not permissible to delay it. Delaying it is a sin that necessitates repentance, as Ibn al-Qayyim stated. A Muslim must repent with sincere repentance and firmly resolve not to return to the sin. If he stumbles and returns to it, he should not despair but repent again, for Allah accepts the repentance of His servant as long as he repents to Him.