"It all depends" is unfortunately the answer. Depends on loco, manufacturer, traction tires (or not), well balanced & properly weighted (or not), if the grade has good transitions into and out of it, if the loco is properly maintained, if the grade is on a curve, if the track is dirty, if the consist is properly maintained and weighted, etc.
Best thing to do is to get as specific as you can - loco type and manufacturer, and the proposed grade (and curve if any) and see if anyone can give you information from experience.
But the general rule of thumb - all else being equal - is that 2.5% is about as much as you'd want to try, unless you are modelling logging lines with geared locomotives.
Andrew