How do I find out how long it takes letter mail to get from one location to another?

Call the post office and ask, they will know.

Go to http://www.usps.com/tools/calculatepostage/welcome.htm?from=home_header&page=calculatepostage, select Domestic, International, or Business; then fill out the Postage Price Calculator form. FYI: A letter you'd normally drop in a mailbox is considered First-Class Mail and usually takes 3 to 5 business days. Hope this helps.