I'm finding that it depends whether they are shipping via UPS, FedEx, or USPS as well as the shipping origin. UPS is typically the longest unless they are shipping from a store, and generally 1.5 weeks. FedEx delivers on Saturdays and takes about a week. USPS is often the quickest and supposedly will deliver my scarf tomorrow (8/7) after the original estimate said the 17th, and estimate was in-between.
I have learned to check back and click through the delivery estimates every few days. I am also a bit embarrassed to admit that I know all this.