How long should the sinner seek forgiveness, and is it sufficient for him to seek forgiveness with a present heart once, or for a specific period before he begins his deeds?

You are not sinful for what you did involuntarily due to obsessive thoughts. What comes to mind in terms of thoughts and imaginations is not a sin that necessitates repentance, because Allah has forgiven this Ummah for what they think, as long as they do not act upon it or speak of it.

If you commit a sin, repentance is obligatory by giving it up, resolving not to return to it, and regretting having done it. If these conditions are met, then it is a sound and accepted repentance.

There is no need to constantly occupy your time with seeking forgiveness, although it is good and commendable. However, you must strike a balance between remembrance (dhikr) and seeking forgiveness, and your daily activities and studies, so that your worldly affairs are not negatively affected. What is obligatory is repentance with its conditions.