Challenge 120

The fire in the rope

Difficulty level:

We have two strings that do not necessarily have the same length. If we set fire to the end of either string, it will take exactly 1 hour for the fire to reach the other end of the string. However, the fire will not move at constant speed, it may be faster at some points and slower at others. The speed of fire does not depend on the direction it walks on the rope. How could we measure 45 minutes with these strings?