I never reached that limit with Blackhorn but i did get to a point with Reloder7 in my Pacnor 45 in which loading was noticeably harder with sabots. I consider it one of the dirtier SML powders and i do go several range sessions without cleaning them. Sometimes the bores are left uncleaned for a few months.
IMO over one session it would probably never happen depending on weather conditions. Fouling that has "aged" long enough might give a totally different result.