He actually has an article where he lays out how he went about saving before he retired, and he basically had $800k in liquid cash/investments before he retired.

But I don't think he actually saved the full $800k; he probably had quite a bit less than that, but with the power of exponential growth, looked at his statements one day and had $800k. It sounds like he also makes a bit of money doing odd jobs throughout the year, too, and since the $800k was more than enough for them to live off of in the first place, it's grown quite a bit since they retired. I doubt he's put any actual numbers out there, but he's hinted at the fact that he's got several million dollars in his investment portfolio now.

He talks about exponential growth a lot. The problem is, people don't really grasp the concept. For example, if you save $5 extra per week (instead of buying that Starbucks coffee on Friday, for instance), it doesn't really sound like a lot. But if you invest that $5 (MMM suggests index funds, which are pretty low-risk/low-growth -- so we'll use 6% annual growth here) you actually end up with about $3600 at the end of 10 years. Sure, this isn't a lot, and most of the growth occurs near the end of the time period, but it's $1000 more than it would be just sticking it in a savings account. MMM advocates a whole bunch of these little changes in your lifestyle, which add up over time.

While he does take it to the extreme, he states often that he's not telling people they need to do what he does.