Wow, in Access that could be complex depending on what you tie it to. In conjunction with the "On Timer" event and a timer interval, you could monitor the "on dirty" event for the record; however, if you use unbound forms, that may not mean anything. So then you'd have to tie it into a "key press" event and/or mouse movement event. The question is, how much resource overhead is this going to cost you when you start doing that?
Michigan Dept. of Environmental Quality, Air Quality Division