I don't believe there is a consensus, but would say that for models in the popular scales:
Grades around 1 percent are minor and don't have much effect on train length.
Grades around 2 percent are moderate and have a noticeable effect on train length.
Grades around 3 percent are steep and have significant effect on train length.
Grades around 4 percent are very steep and challenge the pulling power of locomotives. Trains will be short unless extra locomotives are added.
Grades around 5 percent or more are.extreme and greatly challenge the pulling power of locomotives. Don't use them but for exceptional situations such as for an industrial spur where a single car is taken to an elevated structure like a coaling dock. Keep raising the grade and eventually a locomotive won't have enough traction to go up the track.
Mark
Another excellent reason to hang around the Sears Tool area.
I love that place.
Mmmmmm..Power tools
Blacknight.....
Check!
The other day I purchased an 8-inch "digital torpedo level" from Sears on sale for about $25. It shows grades by angles, inches per feet, and percentages. I purchased it for making the benchwork level and to maintain the desired grade of the roadbed when constructing the layout. It is sure to be handy. It has bubbles too, but who needs that?!
You have a perfectly good answer in grizlump's, but I wonder if there is something more to your question.
As was said, a grade is a grade is a grade, irrespective of scale. What makes your choice of scale reasonable is whether or not your engines can get anything up the grade. If the engine can barely get itself and a tag-along shadow up the grade, you should probably consider reducing it by about a percent and a half at the very least if you want to get a few cars up it. So, in addition to the answer above, you must, or would be strongly advised, to do a series of mock-up trials to test your equipment. Lay a level section of flex track and have the last foot of it rise up to a 2% grade over 12". Add another section or two at grade and stabilize it well so that you can run a train up it. Hook up power and test out both your vertical easement (it may be too short for the engine!) and the engine's capability on that particular grade. Add or subtract difficulty as you wish to determine what you must live with.
grades are expressed in percentages. the formula is the same for all scales. divide the amount of rise by the distance of the run. example; you want to raise the track 1 inch in 50 inches of run. 1 divided by 50 equals .02 or 2%.
grizlump
What is the heigth distance ratio for a grade on a HO lay out?