How long approxamantly does it take for euthanasia to go through a senior mix.?

She is 13, going on 14, this is her last afternoon, she will not live to see 8 pm. she is in much pain. My family has decided to put her to sleep. My question is For a dog that is half GSD and half Chow-Chow, how long from the shot is adminnistered to when she will take her last breath and be deceased?