How Quick Can Fin-rot Occur?

[James Bond the fish]

So about three or four weeks ago I introduced a blue betta into my tank, along with two females. He had hints of red on his pelvic fins (I'm sure this is colouration), and the tips of his tail were tipped with redish streaks, which may have been bloodied fins, but I bought him thinking it was colouration. He hid away and sulked for a few days in a root, but over time he started to poke out more and more until you always saw him somewhere around the root, seemingly happy enough. I don't recall the condition of his fins but I believe the redish parts disappeared.
Yesterday I added a couple of baby angel fish (my giant black widow female is slightly larger than either). I remember the betta showing off with one of the angels, all his nice blue fins erect and shiny, the angel didn't seem too impressed and slowly found something better to do, although I did notice that the back of his fins were a greyish colour. I've just gotten home today... and poor Mr. Man seems to be a shadow of his old self. He's gone from a brilliant metallic blue to a dull brownish blue (my female betta looks nicer...), half his tail fin has almost entirely corroded away, with what looks like hair trailing away from the fin (the bones) and he was sulking towards the bottom, though not entirely inactive. I've placed him inside a fry-trap for livebearers with the bottom removed, put a bit of plant in with him and treated the entire tank for finrot, though no other fish seems to have caught it, and they all seem very happy. There's been no sign of hostility with the angels, which to my knowledge don't grow aggressive until they reach adulthood, and apart from the occasional nip from a black widow, nothing's been hostile towards him.
So my point... is it uncommon for a fish to become ragged and so pitiful looking in less than twenty-four hours?

 

[stang1]

Very possible. Males should never be kept with females, only for breeding as they will more than likely attack each other. There's a good chance that the females have nipped at him and that's what has caused the finrot.
It also sounds as though he is stressed out in the tank, hence the losing colour and sitting at the bottom.
You really need to put him in his own tank and start treating with Melafix/Pimafix to help the finrot. Only use half the recommended dose as they are very sensitive. It shouldn't be long before the fins heal and you will more than likely find he gets his colour back.
Please don't put him back in the community tank as the chances are this will happen again.

 

[James Bond the fish]

I've never heard of females picking on the males, only the reverse. My remaining female has never nipped or squabbled with my male, the male would only chase her away from his log. He's only lost his colours today... until then, he was fine.
I don't think he'll make it... his tail is grey all the way up until his body.

 

[stang1]

All you can do is try seperating him and treating him.
Females can be just as aggressive as males so they could well have nipped him. A popular time for nipping and harassment is when the lights are off although it can happen anytime. Sadly, you can't monitor the tank 24 hours a day so there's no way of spotting the culprit.