I did a search to find information about leaving a stinger in for time a 
time period and found this. Interesting the study shows that scraping it out 
is not the best method because of time delay.

http://bees.ucr.edu/stings.html

It doesn't state a length of time but does state that the longer it is in 
the more venom you will get. I usually wait until I see the venom sac stop 
pumping and then pull it out, five minutes at the longest and some times I 
let the stinger fall out on it's own. If I am working bees and not 
appreciating the stings I just pull the stinger out, never have taken the 
time to scrape.

Karen Kimball 

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