Soaking in soapy water for more than 30 minutes is really pointless, because anything that isn't loose by then won't come off without using more aggressive cleaning like acid dips, ultrasonic or steam. Most horns cleaned with just soap and water still have mineral deposits in the slide crooks, BTW.

Unless your horn is fairly clean to begin with I woudn't soak it because I wouldn't want the grease from the slides and the grime from the slide crooks, typically where it builds up, coming loose and floating into the valve cylinders, bell, etc.

I only soak in clean hot water for 10-15 minutes AFTER brush / steam cleaning to make sure I have rinsed away any remaining soap, oil, etc. If the water is cloudy it tells me I'm not doing a good enough job in my first step.

my 2c,