Over the years I've posted a lot of dogs who were supposed to have standing ears after they were cropped (and one German Shepherd whose normally upright ears weren't exactly upright) and in the course of doing that if there was any one question that was asked practically every time it was yours.
"How do you tell when they're done?"
And basically, because there are so many variables that come into play when posting cropped ears (like who cropped them, style, length, cartilage and persistence of posting--the answer has remained the same--"When they're done..."
Considering that I've had everything from the puppy with an extremely long show crop whose ears stood perfectly and permanently after two posting (which meant his ear posting was finished at 3.5 months. To the dog who had one standing ear at 4 months and one that took another 18 months before it stood permanently I don't even try to guess any more.
If I'm posting ears for someone we go from posting to posting and when I think they are done the owner goes home with instructions about taking the ears down first thing in the morning and seeing if they wilt at all through the day--and if they survive the first night down--rinse and repeat--at any sign of floppiness they get reposted. Until they don't need to be reposted.
If the owner is doing the posting they send me pictures or bring the puppy around to see me on a day that they are going to take the ears down and repost--I look at the ears out of posts and either say, "Needs more posting." or "He's done!"
There just isn't a good formula for how long it takes. If you think of one let me know.