It depends on the ammonia concentration.
Some low ammonia ones can be left in but most of the stronger ones (like Sweets) are strictly a 10~15 minute proposition as the steel of the bore can be attacked by high concentrations of the stuff.
One of the reasons I switched away from them was this. There are a couple (& only a couple) of non ammonia based ones that remove metal fouling chemically like then KG big bore & the M-Pro-7. I regularly leave the M-pro in overnight & have zero problems. That, the lack of a strong smell & the water based cleanup of brushes, jags & so on have lured me away from the more traditional oil & ammonia types.