We spent 1.5 hours at each of the 3 sites, so 1.5 x 3 = 4.5 total stars. :)
Our guide warned us at the beginning that this tour is just an overview of these sites.
So if you're the type who likes to leisurely read every item of information displayed and to listen to everything on the audioguides, you will need a different tour (Perhaps 3-3.5 hours for Windsor Castle and Bath each, and 2 hours for Stonehenge).
No hot food or drink is allowed on the bus.