Does it matter who the thoughts were about, or how long they lasted?
Not necessarily
how long they last, but whether they were given consent of the will.
Committing the will to the
desire contracts the evil nature from the sin one expresses with the will the desire to commit. Note that this is not merely a temptation, or a vague passing wish not truly willed, but a consented act of the will towards desiring to commit that sin. In a case like this any details affecting the nature of the sin would need to be confessed-- for instance, if one explicitly desired to kill his father/brother/whatever, or other such examples (i.e. adultery, incest involving a parent/child vs. otherwise, homosexual behaviour, or any combination of different sins).
For thoughts where there is no explicit desire but rather pleasure obtained from them, I would think a detail that changes the nature of the sin contained in the thought should need to be mentioned if it was clearly consented to specifically, but otherwise just saying “I indulged in impure thoughts X times” would probably be fine.
From Fr. Prummer’s
Handbook of Moral Theology:
[A] person who takes deliberate pleasure in adultery sins in a different way than if he were to take delight in incest. However, in practice, it is normally sufficient for the penitent to confess that he had so many thoughts against chastity without mentioning the other circumstances. For such thoughts (which are not accompanied by an evil desire) do not present their object clearly with all its circumstances; they usually pass quickly through the mind making it morally impossible–especially after a long time–to reveal in confession all their circumstances.
I’m not entirely sure what to make of that, but it would seem to me that if one is fixating specifically on a certain sin and deriving pleasure from that thought, it should be mentioned what it is (however not in great detail-- just what alters the nature of the sin). But I’m not really sure.
This is of course provided that one is actually consenting to the thoughts, and not merely being tempted. Perhaps talk to your priest for advice on this.