How often? Usually when I have six or so 8x10 negatives. Why?
Each negative will take three, four, or five sheets of paper to investigate so a single set of dev/stop/fix trays will do the batch.
My archival washer has 24 slots so a six negative session will just about fill it up. A 45 minute wash then produces enough paper to cover all my drying screens so there is no easy way to handle more.
I don't know if I work particularly slowly but it takes about six hours to do all this and clean everything up for the next session.
Most importantly working through six negatives makes me darn tired. I could do more, have done more, but the quality goes off the wearier I get. Materials are too precious to use prodigally. Working tired means more throwaways the next morning.