While it can be safe to insert a butt plug and leave it in for a long period of time, there can be a problem as the lubricant dries up. In particular removing a “dry” butt plug can be painful, and if you don’t replenish the lubricant occasionally it’s possible (although not inevitable) that you could do some harm through tearing. (quoted from web page given below)

you know now that you mention it I've been meaning to look this up.. I have a rather funny story about it ask me and I'll tell you about it some time!
here is the site I got this info at
http://sexuality.about.com/od/sextoys/a/butt_plugs.htm