I don't have any idea. It would be tough to know since the arrival of mammals is estimated within a block of several million years, so whatever hypothesis you came up with could be off by that factor.
Remember, the theory of evolution explains the fact of evolution, just like the theory of gravity and general relativity explain the fact of gravity.
We know evolution is fact, we're just trying to come up with the best theoretical mechanism for how it works, just like we look at gravity and know its fact, and we accept Einstein's theory as the best explanation.
Well, as to number of generations, I’m really trying to figure this out for real, not just say something’s true and forget about it.
Human and primate reproduction would require us to take the number of years and divide by 10 or 15. For small mammals, perhaps one generation per year.
I’ve looked at the “evolution time line” that is generally accepted and there is no possible way that enough evolution could have occurred. I actually researched a bit, and found that the actual evolution “scientists” are also unable to support their own theory, very much to my surprise. They simply refer to the “Cambrian explosion” as a possible time period of extremely fast evolution, and beyond that they simply say that evolution must be true, so therefore it is.
As far as the theory of gravity, the force of gravitation local to our environment on earth is observable when one drops an object, so the existence of gravity is without doubt; objects can’t hover without energy. The theories regarding gravitation have to do with mathematically defining the force to reflect a complete understanding of it, and so far there is no theory that satisfactorily describes gravity, general relativity and quantum mechanics in a unified way. Newton’s law of universal gravitation, however, which describes the simple observable gravity above, is called a law because it can be proved through observation.
Your last paragraph, “We know evolution to be fact”... etc., demonstrates the logical fallacy of “begging the question”, the most basic fallacy that all scientific endeavor must overcome in order to rise above conjecture in a bar.