Just another few random data points: I got the email for next Monday's flight (Apr 9th) at 3:22am EDT on Wednesday morning (April 4th). For this past Monday's flight, the email came in at 3:58am EDT on the Wednesday morning before. Going back to a flight on March 21, the email came at 5:47pm EDT on the 15th.

6-ish sounds like a good number. Maybe their email server has been slow the last couple of weeks.