Might be considered a breach of etiquette in some places, but there's no "law" against it.
It represents that the sleeves are too long. That's it. Honestly.
As others said it depends on the school and/or style.
In Uechi Ryu when we do arm conditioning if Gi tops are left on then sleeves are rolled up. But when we're done they're rolled back down again.
After that it depends on the dojo and the instructor if a student is allowed to roll up their sleeves or not.
I roll up my sleeves when I feel like it.
Sometimes I just don't want my sleeve over my whole arm. Simple as that.
It depends on the uniform rule at the school. Some places will allow it, others will not.
It does not represent anything, other than what callsign said.
I have long sleeves because it makes a lovely crack when I throw punches in kata. I roll them up when I spar because the crack annoys me more than my opponent, gives my opponent something to grab and just feels awkward. There's no big deep meaning to it, just personal taste.
It represents that the sleeves are too long.
Might I say well done friend.
if so, what does it represent?
