I hadn't really thought about it until you asked the question, except that I also planned to save until I retired.
I save 30% of my gross income, but I live how I want to live - not extravagant, but I go out to dinner or drinks with friends, I go on vacation a few times a year etc. What would I spend the extra money on? Maybe once I get closer I'll go on a few more expensive vacations, but I have a budget for that in my retirement money, so I can wait until then.
My calculations are based on living at the same level I do now at retirement (adjusted for inflation etc etc). If I all of a sudden starting spending that extra 30%, I'd have to be cutting back again to my current spending levels once I got to RE.
My plan, instead, is to FIRE at 52 with a certain level of savings. If I exceed that level early, I'll just retire earlier!