How long does it take to break-in an oil filter? Same applies for timing belts. Timing belts are made to maintain fixed dimensions throughout their useful life. When it's new it is at the peak of performance. Once it begins to change (stretch) is when it needs replacement.

However, if you did all the other items that should be done with the TB replacement; replaced seals, water pump, etc.; you might give them a couple hundred miles to work in before really stressing the system.

Parts that deserve break-in care are brake pads and those that involve potential metal-to-metal contact in use, e.g., engine internals, that may need some eccentricities to work out.