Most of my LOR writers submitted electronically. Which was REALLY nice. Cuz it's instant. But some of them submitted by mail. Usually a 7-10 day turnaround time with mail etc.
But, this is a busy time for them. Keep that in mind. It takes them a while to get through stuff.
It usually doesn't take that long. One data point I can provide: a recommender of mine sent her letter on the 3rd of the month via standard first-class mail and I received notification of receipt on the 9th (six days later).