The answer is really going to depend on the area you're hunting, wind direction, ect. I've hunted from ground blinds and killed hogs at 12 yards that had no idea I was there and I've been winded in a tree stand that was 30' up, which incidentally is way too fucking high and I'll never do that again.
We've got a ground blind set up right now that's about 40 yards from the feeder. We used it twice last week but the hogs have started coming out later with the recent heat wave and we didn't stay out long enough but we had monster bucks at the feeder that never knew we were there. If the wind is right it's all about sound and movement.
In most cases, you're gonna find that wind direction plays a bigger part than stand height unless you're way the hell up there and even then they'll be able to smell where you walked in. For where we're at, I prefer to setup stands/blinds on the east side of where I'll be hunting, since that leaves the spots pretty much available year round since I know the wind in going to change between north/south and we get a lot of wind out of the west.
When I'm stalking them it's nothing to get within 10-15 yards on foot but the wind has to be right. At that point it's slow movements and no noise. I'll stalk with a round chambered and safety off since even the sound of quietly taking off the safety is enough to alert them sometimes.