We semi-fired on 750k, almost exactly. I still work part time, and DH is open to doing contract work eventually. He was laid off about a year ago so it was sort of a forced semi-retirement, before we were 100% ready.
According to the 4% rule we should be living off of 30k a year, but our expenses are twice that (we're in Toronto, I find it expensive here.) We had a good cash buffer, plus I'm still working part time, so we haven't needed to sell off investments or withdraw from registered accounts at all, and have had some extra to invest, so in a year our stache has grown to about 950k (DH is not an index investor, and had some great returns.) This has given us a lot of confidence going forward, not that the stache will keep that kind of growth momentum or even stay this high, but that year one has passed and we have kept our spending under control and started out with some solid growth. If in 5 years our stache has even the most modest growth, then I think we will be OK.
According to cfiresim we are at 100% success, even if we only earn 15k for the next 3 years. (I would love to fully retire in 2020.) The condition is that we will need to sell our house and downsize (or rent) in about 20 years, which we were planning to do anyway.
I agree with other posters above, that if things go south with the economy then we will figure something out. I'm enjoying semi retirement too much to go back to working full time for any "just in case" scenarios. Optimism is key here, as well as confidence in our ability to roll with the punches and adapt to change. There are many possibilities for safeguarding - becoming more self sufficient with DIY, cutting expenses further, living with a variable income so that on good years we withdraw more and travel, do house repairs, etc. while during not-so-good years we cut back to a bare minimum. My parameters in cfiresim are an annual spend of 60-80k (though the minimal years could be much less, depending where we are living), minimal CPP, and no inheritance, although we will likely inherit something eventually. So I'm fairly confident that we'll be fine - we'll see how I feel during a down market though!