Most of the subtests for 1st grade were in the 25-30 minute range, but they did vary. I don't know if other grades are longer or not, but I would assume they might be.
Also, if memory serves, there's one math one that is pretty short, since they want to check calculation speed and not just accuracy. Presumably some kids don't finish, and the problems get harder at the end of that subtest--double-digit addition instead of single-digit, I think. (It's was close to "above grade level" as the ITBS had.)
I fear you'd have to get your hands on the instructions to get a really accurate read as far as time. But if you just want to see how he does in a casual sense, 25-30 minutes per test is probably safe.
I haven't looked at sample tests online, but the 1st grade test wasn't what I'd call hard...
