I figure a good rule of thumb is to take what you need now and double it (inflation, COL increases etc) then figure how long you expect to live. I'm in my mid 30s. I'd say I could live on $2000 a month if I needed to. I expect to live about 40 more years. So
$2000 * 2 * 12 months * 40 years = $1,920,000
Round it up to $2 million for a little buffer just in case.
