There should be no issues with using masteron for as long as youd like. It's a very nice drug in terms of effects:side effects.
400-600 is not what I would consider a starting does though. Thats very much closer to the high end Ive ever heard of. Maybe by itself that could be a starting dose, but even then, I dont think that's a good idea for someone with no experience with the drug.
Id start with 300mg/wk. You would certainly feel the effects of 200mg when joined with 750mg of test.
You may find that you need less AI compared to what you normally would with the same amount of test.
personally speaking, with 500mg of test I would only need about .25mg of adex every 3rd day or so. With 250mg of mast added I didnt need any at all. Be mindful of that. This offsets the cost of the masteron, for me at least.