Yes, time is the question. If you have plenty of time, soaking in firearm ultrasonic (US) cleaning solutions with the heat on before running the thing can also help get it clean faster. If you don't disassemble fully (and some don't), the final step of running the part in the US (to get it into tight places) filled with water displacing oil is important. I have put some extremely tough jobs (M1A gas cylinder, for example) in other water-base solvents (Slip 2000 Carbon Killer) to soak prior to the US cleaning, but I rinse them off in another US filled with water just before going into the US with the gun cleaning solution in order to avoid chemical mixing to any significant degree. One exception I can think of is after a 3500-rounds-with-no-cleaning function test (yes, it ran that many rounds before the slide stopped closing completely on its own), I once left some heavily fouled 1911 parts in a container of Ed's Red for a week, which turned the bulk of the fouling into a sort of black sludge that was flowing off of them to the bottom of the container. I then took them out and let them sit a day in mineral spirits to clear the other components in Ed's Red, and then let them dry for a day before going into the US solutions (Infante Sonic Tonic gun cleaner and their Sonic Lube water-displacing oil). That made the fouling really easy for the US to drive off.