If your running it slightly rich and taking good care of it maybe 4-5 gallons before you start seeing the compression drop.

You should not open up the engine to remove dirt since there won't be any dirt to get out of there if you have taken care of it. If there is dirt in there your engines trashed anyway If you wanna clean out the grease and stuff WD-40 works very well, take off the glow plug and header you spray (alot) WD-40 in the engine with the piston above the exaust opening. Let it sit for 10-15 minutes. Pull the pull starter while the engine is upside down, it gets rid of most of the grease and oil left from the fuel.