How standard is American Standard Mounting for mortise locks?
Maybe.

American Standard Mortise standardized the size of the opening in the door that the lock body fits into and where the screws are to affix that in place.

However, it seems everything else is subject to variation. The centerline of the knobs or handles may be a bit higher or lower in the new lock than in the old one, requiring additional drilling. The size of the opening for the handles may also need to be different. 

The spring latch, deadlatch, and deadbolt may also be higher or lower on thd door edge. In the simple case this is solved by replacing the strike with the one that came with the lock -- again, mounting holes are somewhat standardized -- but this variability means the new lock may not align correctly with an existing electric strike, which you probably weten't planning on replacing. And I've seen the deadlatch located both above and below the spring latch, further complicating this.

Measure everything precisely before shopping, measure again when you get home, do a test mounting of the new lock body so you can actually see how door and frame will align before you begin any other installation steps, and be prepared to either take the new lock back and try again or have the job Take Much Longer and potentially require additional investment to salvage.

(Been There, Done That... Or failed to do that, because my friend said the supply house said it would work and he'd checked it... my own fault for not checking it myself immediately, but I was distracted by the metal door, which is a set of issues I'm less experienced with.)