“You stayed not but a little, if you had only known! Did you think that We had created you in play (without any purpose), and that you would not be brought back to Us?” (Surat Al-Muminun 114:115)
The Prophet صلى الله عليه وسلم said:
“Strive to do that which will benefit you and seek the help of Allah.” (Muslim)
“No people sit in a gathering in which they do not remember Allaah or send blessings upon the Prophet, but it will be a source of regret for them on the Day of Resurrection, even if they entered Paradise as a reward.” (Ahmad)
So the only regret for people of Paradise will be the time they wasted in this life for any thing other than seeking pleasure of Allah سبحانه وتعالى
Ibn Qayyim (rahimahullaah) said:
“Regret for wasted time should be deep, for time passes quickly and it is difficult to make up what you have missed.
Time for the worshipper is a time for worship and reciting awraad, and for the devoted Muslim it is time for turning to Allah عز و جل and focusing on Him سبحانه وتعالى with all his heart.
Time is the dearest thing to him and he would feel very sad if time passes without him doing what he is meant to do.
If he misses time, he can never make it up, because a second time has its own duties. So if he misses time, there is no way he can bring it back.“