There are dealers like the The Armory in annapolis that follow the law to the letter, which is great. I think the law says the MSP has 7 days, not including the day you buy the firearm (might have the day count wrong, but not more than eight) to "Not Disapprove" you. If you do not receive a response from the MSP within that time then you are free to release the firearm to the customer. The Armory does this. Most shops do not. Most shops will hold the gun until they recieve the "Not Disapproved" or "Disapproved" letter. They don't want to piss off the MSP.
Just for the record I have purchased 5 regulated firearms and my paperwork was always back when I came back on the 8th day.