How often do DD 368’s get denied?

It depends on your unit and state, and a number of other factors like how long have you been a member of the Guard, is recruiting meeting the requirements, are you in a mission critical position, etc. A DD368 does not require the commanders approval, he will just forward it up the chain until it reaches the first General in you chain of command, or his designated representative. I would expect a 90 days turn around at best. It's sometimes faster, sometimes slower. I haven't been active reserve since 1995, but then in Kentucky it was rare for the State to release anyone that hadn't served out at least half of their enlistment. That doesn't always seem to be the case today.

CPT Lawrence Cable is correct. In addition to that, your continued character of service is also a factor. That is to say, your Commander may have signed off on your 368, but if you fail to maintain good standing by not passing APFT, Height and Weight, marksmanship and so on your 368 can get pulled back. I have seen Soldiers get their 368 denied because they became complacent and derelict in their duties because they figured they were in the clear.
Keep doing the right thing (even when no one is looking) all the way up to the end.
Don’t give them any reason to second guess their decision.