Depends on the amount of area you can cover with binos. I grid everything. I usually start at the top, and work L-R, then drop so the bottom of the first view is now in the top of the new view, and glass back R-L, etc. If it is a really wide area, I will break it up with a landmark and do half at a time. But, I always look at each area at least twice, usually 3 or 4 times.
If I am high and glassing flats, I can sit for hours. Amazing how there can be nothing one minute, and a full view bull elk the next in the same location. But, it is usually an ear, rump, legs, antler, color, or movement you spot. If you see something that just doesn't look like it belongs, it probably doesn't. Stare at it and 50% of the time, it is game. Rare to see an entire elk, so don't LOOK for an entire elk. I glass from .5 to 3+ miles out. So many times, I glass past the same spots over and over, and find something I didn't see before, and viola', an elk (or deer) part seems to just jump out.
Also, knowing WHERE to glass is more important. North facing slopes, even on late hunts, has always produced way more game than sunny south facing slopes. Shade/shadows are important to game to stay concealed and stay cool. Pressured elk with move to the thick, nasty, steep stuff away from roads. If you don't want to go there, the elk are likely to head to that spot.
If I am glassing a single hillside, it is a minimum of 30 minutes, or a couple hours if looking at a range of hills. Can't tell you how many times I have glassed up bedded elk by seeing a single ear. Then, you really start picking apart every shadow and tree, and an entire herd materializes, hidden away. Also, move 25-50-100' and glass it again. Amazing what a 1-2* angle change can make in finding game. Most of these were glassed up after changing position by 50' or so.