There are two kinds of clefs that organists most often use: the treble clef and the bass clef.
One is easy to learn, another - not so much. That's because we learn treble clef from the start and the bass clef - only later.
The challenge is that both are necessary to play organ music well.
Nobody can tell you what proportion of sight-reading you should practice with your left hand in the bass clef.
The best advice is just do it every day until it's so easy that you don't think about it.
Kind of like walking...