That’s not how DBs operate.....they quietly get on station and wait to attack when a noisier enemy comes into range. It doesn’t matter really how long it takes them to get into position, either. Again, the DB’s greatest advantage is that no one knows it’s there.
You’re going to have to build a lot of em then to get lucky, why not just outfit a nuke with batteries to quiet them down some??? Once you surface your stealth is gone and four weeks is a small window of opportunity in the larger scope of things. D=VT, 5 knots x 4 weeks is a small distance compared to the size of the seas. 3.1 mi/hr x 672hr = just under 2100 mi. Not much!!