How far in advance can I make reservations

Some travel blogs state that a traveler should make train ticket reservations well ahead of the trip. Is that possible when utilizing the JRpass? I’m particularly concerned about travel in Hokkaido during late November to early December. The lines that we want to use require reservations. I’m concerned about the itinerary falling apart because I didn’t make reservations early enough.

I’d appreciate knowledgeable guidance about how to proceed. We will be in Japan from mid-November to mid-December.
Thank you for your help!