RE: How long will it take for them to change?
We've timbered our property off and on for a good many years. It's being timbered in sections over the course of several years. No doubt about it, it will improve your ground in a hurry. Like mentioned above, it will require you to do your homework all over again.
One thing I have always noticed, from looking at past notes I keep is that the deer patterns evolve as the area that was cut changes. The first year or two, depending on how much sunlight reaches these areas doesn't usually see a lot of daylight activity. As new sprouts shoot up and these areas get thicker and thicker, you'll start seeing more and more deer in them in daylight hours. Deer patterns will change as the amount of cover changes.
After a few years of regeneration, it'll get thicker than heck. Thats when it will be at its best. Lots of cover and quality food. But I'll warn you, its tough hunting when it gets really choked.
Also, when you're scouting these areas, look for the edges where the cutting stopped. Find good trails to and from the cut along the edges.