I think it should just go action day to action day. You start dip, then at the end of that action period the planet is yours. Remember, there is supposed to be like 6 months in between action periods game time, I dont recall diplomatic alliances between nations to ever take over a year, after a couple months its either gunna happen or its not.
So for example of what im talking about:
7/3/12 fleet exits at Bakura begins diplomacy
7/15/12 planet now belongs to RA but not counted for money generation (things just happened, takes time to get stuff in place)
8/1/12 planet is now counted for money generation
Now, this takes just as long to pull off as your stuff already in place BUT it doesnt involve the player having to keep his character and fleet in place for an entire month to do it. We would only have to remain until the next action day, and the second action day for money generation is too keep people from going in on say the 14th or 30th and starting diplomacy to quick gain a money boost the next day.