A long time if you're squatting twice a week only. Even longer to get to 35".
- Squatting twice is a week is fine. Sure some people respond well to three times a week, but for some people respond better to twice a week. By "only" are you implying even squatting four times a week? Again, there's no basis on telling him he can't get his strength optimally up fast if he doesnt squat 3-4x a week. 2x a week works fine for most, if it's not already the best for them.
- He weighs 72kg and reps 90kg 6 times. Provided with correct training + optimum diet, he can get to 1.5x+ BW squat fairly quickly I would say. Of course like I hinted at earlier, everyone is so different so no way of telling. At over 1.5x BW with a low BF as he has (6%), he should be able to get into high intensity plyo's provided his overall fitness is fine.
- 29" to 35" is a lot. You still never know though. With his weak relative strength which can potentially go up quickly, and assuming he is a double leg jumper (not saying this won't work for single leg jumpers), and being only 17 years old and potentially having his body go through more maturing, in the best case scenario he can get those 6" quickly. Not saying it will happen, but don't just shut it down so easily.