I have a GMG Daniel Boone and a Davy Crockett.  Do you ever run the grill at high temp just to burn off the grease and crud?  If so, what temp and for how long?  I was thinking about cranking up the DB to 500 and letting it go for 3 hours.  Is that too long?

Never done that, I have run it up to 350-375 and let it go for 1/2 hour or so but I keep mine pretty clean. If it were me I'd get some more responses before but that is just me. I have run them higher plenty to cook but never 500 for anything.

Personally I never burn off to clean my arsenal of pellet smokers.
Give it a good scraping with a putty knife, mix the scrapings with the ash and vacuum out. Now wash the grates, you'll be good to go. I wash the grates almost every time before cooking.

I had a soot issue and GMG grill support told me to do a high burn until (excessive) smoke is gone. I do scrape drip tray and vacuum out fire pot every other cook. I find that if you do a lot of chicken or other high grease cooks you may want to do the burn off afterwards. I always scrap and vacuum before the burn off. Then I start around 375 and up the temp by 25-50 until I get to 500. I never go strait to 500 because I don't want to start a fire. Not sure if it would just makes me feel better. I have left mine going for 2-2 1/2 hrs with no issues. The more you do this the shorter the time will be. Most of the time it is 1-1 1/2.

Mudflap

I usually try to keep my pit scraped & clean. After my recent fire in my DC I think I'm gonna do some burn offs once in a while for further cleaning.

I did the burn off.  The grates and drip pan were greasy from turkeys and bacon.  I first scraped off the drip pan, and then fired up the grill  I left the grates and my GrillGrates in the grill.  After it hit 150 I just cranked it all the way up to 500.  It went up to 528 and then dropped back down.  It varied between 500 and 509.  It smoked a lot of white smoke for about 1 1/2 hours, then the smoke decreased.  What looked like black motor oil dripped from the chimney cap.  I kept it at 500 for 3 hours.  I refilled the hopper twice during that time.  It definitely burnt out the grease, and left white ash on the GrillGrates and drip pan.  What was left was crusty and brushed away clean.  It was very similar to using the Self Clean mode on our home oven.  Tomorrow, I will pull out the grates and drip pan and vacuum it out.

YMMV, but the burn off worked for me.

I need to do a burn off on all of my grills this week and bring most of them back in the garage for the winter.

 Looks as if you got it right  dankirk..To lessen the burn off time, try and keep the pit somewhat clean after each cook, and before a burn off a good scrap down making sure to do the bottom side of the grate(s).

I had never done a high temp burn off until a support rep for my CS450 suggested it for some performance issues I was having. All it did was cause a grease fire that ruined the thermocouple. So, I don't do that anymore...

It eventually runs into the bucket or burns off over time anyway, and saves some pellets. That's my opinion, anyway...

 Forgot to add.  If doing a high temperature cook, make sure you do a burn off first. Otherwise the food will take on a very distinct off taste.

I'm not too concerned about my grates, but I do try to keep my mats clean and running the grill after each cook at a moderate temp (375 or so) does that for me. It makes their cleanup much easier.