How long does support typically take to respond here?

quite a long while, if at all. they're really swamped with tickets...

anywhere from 30 minutes to a year ~

It depends, if requesting a new winner for a giveaway it can be done quick. Any other ticket can take years.

Wow, reminds me of our buddies over at Valve. Oh well, nothing I can really do now except wait. Thanks for the help.

forever

Support can't do anything about fake negative rep. Only Super Mod can and there's only one super mod.

Well.. :D
here it is

The best course of action to take when you get -rep like this is adding the person directly and try to get them to understand they where scammed by an impersonator (which i'm assuming is what happened based on the -rep)
If they do accept your invite and the scam happened not too long ago have them dig through their activity feed, it should show that that person became friends with another account with the same name before, most likely private as well.

If that doesn't work try to forget about that -rep and don't let it discourage you to trade, once you have a few +reps after that -rep most traders will understand what has happened, don't count on support to remove it anytime soon in any case.
Like Jatan said before me there's only 1 person who's able to remove the -rep and in my case it took about 9 months but this was back when there was a huge backlog, response time is probably a lot better by now but still I wouldn't count on getting it removed within a month or so due to the sheer amount of people using the trading section with only 1 supermod.