What will get really interesting is this year is the first year hiring is kicked into record rates, across the majors.

A third of the pilots resigning for other opportunities? I bet that increases as the big vacuum sound you hear is the majors sucking them up. Repeatedly, for each year over the next decade, year after year.

This is why the flow will stay between 4-6 years for a long time. Flow+attrition+retirements

how much attrition typically each month?

That depends on what part of the seniority list you are talking about. The bottom one hundred on the list see 9 people leave a month outside a 6 per class or 5.5 per month flow.

The most senior 200 see barely one a month outside the flow. It all but stops for the year before flowing with one person per year attrition outside the flow.

4 years is an outright lie

as said previously telling a new hire they could flow in 4 years is laughable. Dec 2018 hires are still looking at 500+ people ahead of them not including lifers and they have now been on property over a year. So to suggest they will flow 3 years from today is silly.