Good day all, what's the expected duration for a typical new hire to be reserve duty? Apologies for any ignorance on my question I'm to all this.

Once you complete OE you are a line holder. You bid a line and get awarded according to your seniority. The line itself can be Primary, Reserve or Secondary.

Corrected.

I think reserve is pretty clear and I suppose primary. Could someone clarify the others?

Secondary lines are basically super long call reserve 2 days out. VTO isn't really a line type but a second bidding process to bid for trips dropped for vacation and training consisting of the other 3 line types. If you want more information the contract is on iap2750.org. No log in required.
As for when you can hold primary lines depending on the fleet it looks to be about a year or so. However on the 76 reserve lines tend to go way more senior.

New hire 747 FO ANC hold primary lines.

I'm on the 74 and have never had a reserve line in 2 years. Always a primary line.

How long is the 74 course and where do they do the training? tia

All training is done through our Miami Training Center. Some fleets may utilize Boeing in Miami or other small simulator facilities in the area, but Miami is training central.

Please read the multiple threads discussing training and current delays. Due to various reasons, two months of training is taking upwards of five months. On all fleets likely, and possibly longer. You will be on year one salary at min guarantee from date of hire, so don't worry about pay. While in Miami you will also receive per diem. If you are sent home after indoc awaiting ground school, you only lose per diem.

Read the Atlas Air Hiring thread if truly interested.