Our school has a Process with a capital P to request letters officially, and students can’t start that Process until senior year starts. Kids in the know (those with older siblings, mostly) do ask the teachers informally at the end of junior year, so some teachers start them over the summer while some wait for the Official Request.
Our teachers do also get release time based on the number of letter they are asked to do, so that should make it a bit easier on them, I hope.