I don't think that there is always a set time that you can go for just automatically - I think it depends on the child and the activity time and level and so on. For instance, when younger my DS did 30 minutes for an OT nap, and 45 for a UT. As he got older his UT naps could be anywhere from 37-50 minutes. As a toddler he does 1hr20 as OT and 1hr45 as UT. But I know a lot of moms with children the same age would say that for them, a 1hr45m nap is FANTASTIC, neither OT or UT, just a great nap. But for mine I know that it is NOT a great nap and he was def UT.
So I think it changes by the child and the age. But I also do know what you are asking.
At 10 months I wouldn't judge a 1hr15 minute nap as either without knowing what the routine is like. For instance at this age LOs are probably not taking 2 naps of the same length...one is probably quite shorter than the other. So if both are 1h15m then I would say to shorten one of them and the other will lengthen out. But if one is less than 1.5 hours and the other more than 1.5 hours then for a 10 month old I would say it is neither UT or OT but pretty much bang-on!
I would hazard a guess that 35 minutes is OT.
So if your LO is doing both of those and on the same day, what I would think is that they are prob both OT, possibly the 1hr15m could be UT or at least could be lengthened out by a longer A time (unless it is the afternoon nap in which case I would say it is OT after an OT morning nap...) Prob sounds confusing, but do you see what I mean about not being able to look at times in isolation? I don't think you can really diagnose them without looking at the full day, kwim?