It's only guesswork at the moment. The facts are the Kickstarter estimated one month each for the alpha, premium beta and standard beta stages. You would therefore think that each of these stages would last at least one month, otherwise the people who backed at that level will feel shortchanged. The Alpha actually lasted nearly six months, so we know they can be a lot longer than a month.
One thing I will say is that I would guess the end of premium beta will be telegraphed in the same way that the end of alpha was, and as with alpha there will be a cut-off date for new orders which will be a good month before the the end of the phase. A fixed cut-off date is good for everybody: for customers it solves your information issue, and for Frontier I would imagine it is a good business decision.
I personally would guess ~3 months for the premium beta phase.