Lets say we have two identical clones(growth, nutes, enviro etc).
From the same mother? I would not expect to see much variation if all clones are the same age, we re rooted the same way (and to uniform results), subjected to the same stresses and the same environmental, nutritional, and light (color + intensity + hours), et cetera.
Fully rooted they stand 5 inches tall.
Clone1 is allowed to grow untouched, clone2 has training done to lower the height and widen the canopy.
After 1 month of veg, Clone1 is let's say 20 inches tall, clone2 however is only 12 inches tall.
Well, you are no longer talking about identical clones. Plants with different orientation - and height, generally, for indoor lighting - are no longer receiving exactly the same amount of energy. Training (vs. untrained) moves certain auxins around, which change which part(s) of the plant get the most energy... Again, they are no longer identical clones.
Their general growth characteristics ought to be close, again depending on how close the two plants are. But both plants are no longer the same size/shape/mass, probably... Has the plant size to root zone / container (medium) volume ratio thus changed? Etc. But things ought to be close. General length of flowering should be pretty much spot-on, so I'd guess that the stretch period would also be identical in length or at least very close. Rate of stretch... I could let you know at harvest time, if you've kept your log up to date, lol.
Do you know the exact flowering time for the clones' mother? I could predict the length of the stretch period from that (given a reasonably normal grow).
Both are flipped to 12/12 and stretch 3x the original size
Is that an assumption, for the sake of argument, or have you flowered clones from the same mother before? Because the amount of stretch can vary both by strain and among different phenotypes of the same strain. You can even see some variation in stabilized inbred lines (IBLs), although that'd be more like a bell curve, I think, with the bulk of the population falling between a relatively narrow range.
Will clone1 stretch to a total height of 60inches, and clone2 stretch to 36 inches? Or will they both stretch to 60 inches?
Are you going to cease all training of the formerly trained clone? (...and return it to a vertical stature in such a way that it once again matches its sibling?) If not, that'll be a change, too.
In other words, does the stretch go based off actual height, or actual growth? As in, clone2 would have grown to 20inches had it not been trained.
Again, I would expect the stretch period to be similar in length, but the rate of growth during the stretch period is like... well, like the usual rate of growth, lol. In generalities, it is governed by strain and phenotype selection. But just about every change is going to affect your rate of growth in some way - either as a positive or a negative - even if the amount of change is very small. If you want to know the answer to your question, what you do is to pay attention to what you do, how you do it, how the plants respond, et cetera - and keep track of this data. Then, after harvest time, you can look back and answer that exact question - but only for that particular strain. Really, for clones of that mother, lol, but for that particular phenotype, at least.
That's why people who grow the same strain multiple times tend to increase their production by at least a small percentage over time. They can predict with reasonable certainty how the plant(s) they're growing will perform best (and how it can be expected to perform under less than ideal conditions, which may be... useful knowledge if there is an issue in the grow room that must cause a compromise in the grow).
BtW, clones tend to flower in slightly less time than their mothers. That would indicate that the stretch period would be shorter in the number of days, too.