Union Business in the Library

I understand that, as employers of a union shop, the library is required to allow the union a reasonable amount of time to conduct union business.[1] However, how much time is reasonable? Can employees meet with their shop stewards on the clock at the library for 5+ hours a week, taking both away from their duties? Is there a ballpark? Could we set a limit of 1-2 hours a week?
Thank you!