If you make a reasonable offer, usually something will go within a couple of days; however with Black and White out in Japan, a decent chunk of the userbase is gone, so it may take a little longer. Also, people can only ask for things they have seen, so trying to trade rare things that aren't hard to get in the endgame sometimes doesn't go as well as just offering a Vulpix or something else everyone thinks is cute.

If you can generate Glaceons or Leafeons they will go like hotcakes, since people with only SoulSilver or HeartGold have no other way to get them.