We aren't actually given a specific time that Maniac took to untie the knot, however it is clear that he takes most of the day to achieve his great feat. Regular reference is made to the passing of time during the day, such as "the long August afternoon." Of course, Maniac Magee is finally successful in accomplishing his great feat at dinnertime, we are told, when the narrator joyfully announces that:
Cobble's Knot was dead. Undone. Gone. It was nothing but string.
We can thus infer that Maniac Magee started this immense challenge at some point in the morning, and, apart from the fifteen minute nap that we are told he took, continued at it without a break until dinner time, spending about eight hours on the task until his success.