I started farming after my uncle got done with me, in 1969. First year was a bust with dry and little crop. I stuck with it. Had some pretty good years in the 70s, that were punctuated with the Russian grain deal. Then the 80s came and for ten years, I lost money but was somehow able to remain cash flow positive. So the answer to your question is; ten years. 
I had a goal in mind, so was willing to endure short term pain, with the aim of building a farm.