Look at these two sentences:
I've planned my visit to Japan for years. I've been planning my visit to Japan for years.
To me, both are acceptable and interchangeable. Would I be incorrect? Both of these examples use "for" (duration), indicating an unfinished timeframe.
Edit: My reference is Michael Swan's Practical English Usage, page 421, where he says "Generally, however, both progressive and simple tenses are possible like these, with a slight difference of emphasis" 
