Is there a reg that states how long a unit has to get drill dates put out to its soldiers?

That's a damn good question

There isn't a regulation, explicitly.

I'm going to take a wild guess that your schedule is posted at the start of the fiscal year, and multiple revisions and modifications are made to it subsequently that aren't published and disseminated, causing attendance issues and employer conflicts?

From experience...all you can do is confirm it every single drill. An effective best practice I've seen is a unit newsletter made by the UA every month, confirming the date, that everyone is required to receive on their civilian email.

I am sure that the Oregon Guard has a guideline that covers how long it takes after the YTC is generated before the drill dates are published but I can't recall an actual AR that covers it but when I was commander it was one month after the ytc was published that all soldiers had their drill dates. Now, in the Army Reserves as the assistant S3 we have one month to publish all BA dates before BDE jumps in my ass.