The standard way to develop a film when you didn't know the development time was to do a clip test. In the dark, you cut off the first five or six frames, develop them at your best guess at the dev time and see how they come out. If they were okay, you develop the rest of the film for the same time. If they were too, thin/heavy/contrasty/flat, you altered the dev time accordingly. A standard, commonly used technique. All pro labs would offer that as a service.
Alan