The simple answer is as long as it takes for the blockage to come loose. Get a 1/4" to 3/8" diameter long rod that is longer than your barrel. Grind a point or a chisel edge. After your breech has soaked for a couple of hours, set the drained breech on a wooden support and give mass of shot a few firm taps. Rotate the barrel a quarter turn and tap again. Turn the barrel to pour out any loose shot. If you get it all out use the rod to break up the powder. If you are using MAP (Murphy's, alcohol and peroxide) the powder should be loose enough to be down out by compressed air. You can use a patched jag to flush out all remaining gunk with the breech in MAP. Do you normal cleaning.
If the shot isn't coming loose after soaking for a few days, then it's time for the extreme method to remove the breech plug. Based on your pictures, you have a snailed breech that is hopefully threaded to your barrel.
Be sure there is an index mark on the breech plug and the barrel for realignment on reassembly. There have been some of these low cost guns that welded the breech to the barrel. Judging from the fit I see in your photos, it should be threaded. Now all you need is a sturdy, heavy work bench with a large bench vise with protected jaws. Get a cardboard tube to use on your barrel. You will need a wrap of leather in the jaws of a very large adjustable jawed wrench around the breech plud where you can grip it. Crescent is the brand often used. You may need a long pipe for extra mechanical advantage. Once the plug is out, the long rod can be used to drive out the blockage.
Clean it up and reassemble.
ne part alcohol, one part hydrogen peroxide, one part Murphy's oil soap. This mixture has been cleaning gumped up black powder guns for over fifty years.
