Dumbledore doesn't give a straight answer when Harry asks if more than one time turner can be used to get more than 30 hours.

There was another slight pause, during which Harry went on smiling. He was a little apprehensive, actually a lot apprehensive, but once it had become clear that Dumbledore was deliberately messing with him, something within him absolutely refused to sit and take it like a defenseless lump.

"I'm afraid Time doesn't like being stretched out too much," said Dumbledore after the slight pause, "and yet we ourselves seem to be a little too large for it, and so it's a constant struggle to fit our lives into Time."

On the other hand, we may infer that thirty hours is the limit from e.g. Amelia Bones' behavior in the Azkaban arc:

"I'll check if we have anything from six hours forward," said the voice of Madam Bones, "if so they wouldn't have told me, but I'll have them tell you. Do you have anything you want to tell me, Albus? Which of those two possibilities is it looking like?"