I haven't hunted burns a whole lot, but the couple I have, this is what I observed:
Welcome Creek Wilderness, the burn was about 4 years old. Lots of grass and small saplings coming up. Quite a few elk in there during mid September. Also, lots of black bears and some deer. The thicker it got over the years, the fewer elk there were. I think that burn is about 12 yrs old now (?) - less grass; blue stem and good grazing grasses are largely taken over by bunches of bear grass. Elk are rare now - seem to have some move in once the snow starts to fly, but not that many. Still some deer and bears, but they were probably there before the burn.
Pioneer Mountains - In an area you have to hike to, well off the beaten track; an area I know well and know it to be full of rutting elk in late September. I was there a year after it burned and it was like the land of Mordor. It was really kind of depressing to see it that way. There were no elk, but there was also no feed - yet. I'm planning to go back next fall, but It's a hard place to hunt w/o a partner. It's a long, difficult, multi-trip pack when you kill something. My good friend and hunting pard moved to Anchorage, so I've pretty much been on my own the last couple years. I anticipate it will be better next fall, but I guess we'll see.