Re: How long w/ Coach
Very difficult questions to answer, perhaps impossible to be specific here.
If you are seeing better growth with a coach than you were previously and the money is a clear positive investment, sticking with the coach while those conditions remain true makes sense.
Now, sometimes players are really bad and need to kind of be broken down and taught a lot of basics they never learned. Some players will lose confidence while being taught and the biggest of all: some players will automatically assume the coaching was really good or really bad due to short term variance.
I know players that have worked with coaches for over 3 years. I don't see a major pattern there, some players probably are wasting money on their long time coach and some are getting great value (in the situations that I know of personally that I'm thinking about now).
If you're a micro player, you might consider watching and exhausting video training of any solid SH or FR coaches out there first. Those are pretty popular games and there are a lot of great players that you can likely watch and get into their thought process and approach to playing the game without it costing you too much money (compared to private coaching).