I have 50 32/40 cases that have been reloaded with BP about thirty times. They are holding up very well. I boil them in dish soap and water soon as I get home. Clean them with a bronze bore brush when I rinse them in the sink.  I dry them in the pan used to boil them in, on the stove.

Dont re size my cases. I'm now using .60 Veg wads to hold the powder in.

                  Joe.   

18 years ago I bought 500 38/55 cases for my CPA kept 350 for my fixed ammo 38/55 sized 150 down to 32/40  of that 150 use 100 for gallery fixed other lot of 50 is for breech seating.   38/55 runs about half the time on Black 32/40 always Smokeless.  Either one clean them good after each use.  Soap and water to clean the black powder cases  then vibrate, just vibrate the smokeless cases

Breech Mostly work off a box of 20 and had the other 30 in reserve until I started breech seating the 38/55 some matches. so expanded 20 of the reserve cases back up to 38/55. Shooting the Hudson match I use one case until the primer pocket gets dirty and hard to seat then start a fresh one generally 3 or 4 cases in 100 record shots and perhaps 50 sighters. Switch cases always go to the sighter target to prove the new one.

Way I run the 38/55 fixed is 6 MTM 60 round boxes 3 Green 3 Red.  Load 3 boxes one color as a lot all the same time, shoot through them  until almost empty then load  up the other lot.  Two colors makes sure I don't go to a match and use ammo loaded at two different times. 

When starting a lot of 3 boxes always re-check my zeros to make sure elevations are the same.  Accurate is important but always shooting to the same zero is more important with most Fixed Ammo matches. Sight in times are short and best used to read the wind not make sure your ammo is OK.  Lost some cases over the years but still have well over 150 cases in each lot. I expect them to out live me and I have no plans to go anywhere

Boats

Have you tried the ultrasonic cleaner without the salt in the mix? Am not a fan of chlorine (NaCl) from the salt. Almost impossible to completely remove it when rinsing. Think I'm going to ask for one of those $99 ultrasonic cleaners for my Xmas present. From what I've been reading they are fsater and save some work on the primer pocket.

I use a mild citric acid solution, no salt, and a drop of detergent - works fine.  As far as I know, the salt doesn't do anything.
Oh yes, and as to the original question, case life with BP:  if you use fireformed cases, don't size and anneal once in a while, essentially they will last till the primer pocket wears out - that's the 1st thing to go...

Have been looking at various ultrasonic cleaners in the <$100 range and most have timers that only go up to ~5-6 minutes. How long do you need to run them in order to get the BPCs clean?

After looking at the Lyman, I'll likely give one a try. They only go up to 480 seconds (8 minutes) and from reviews I've read, sometimes have to run two cycles. On sale at Midway for $95 till 11/30. can get considerably cheaper at Harbor Freight, but prefer to have the Lyman warranty as they generally stand behind what they sell. I do get a kick out of the folks who purchase them and try to clean with straight tap-water and then write a bad review.

A 5-litre unit with heater would be close to perfect. I use a 20-25min. cycle for my BP cases at about 50-60 degrees celsius.