I am wondering when does not praying become a sin? is it a mortal sin to not pray daily? or not praying enough or imperfect prayers?
One answer that we know for sure is the failure to attend Mass on Sunday due to negligence. That would be a case when not praying is a sin.
Apart from our Sunday obligation, however, there is no standard or measure for how much prayer is required. Remember "prayer is the raising of one’s heart and mind to God” (CCC 2559). This is something that we work to make a habit, but there is no quantitative line that determines a deficiency that is to be considered sinful.
Is it possible that a lack of prayer could become sinful? Yes. If you develop a habit of not acknowledging God in your daily life, then you could be guilty of sin again the virtue of religion and piety. But that line is hard to determine.
I would encourage you to continue putting forth the effort to develop the habit of prayer everyday, which could include both informal and formal prayer. But if you happen to miss your routine formal prayers, don’t beat yourself up. At the same time, however, you don’t want to dismiss routine formal prayers.
If you forget to acknowledge God informally throughout the day, tell him you’re sorry and begin to do so.