How far out can i book?

Some trains can already be booked, e.g. Eurostar, ÖBB Nightjet, DB. Although there's no need to book DB so far in advance. Interrail is often late showing the new annual timetable, which may take another month or so. Seat61 provides an overview of approximate booking horizons:

https://www.seat61.com/european-train-booking-horizons.htm

Seat61 also has a page about where best to book reservations:

https://www.seat61.com/interrail-and-eurail-reservations.htm

Thank you for your comment. My way of traveling is booking everything about a year in advance. I fly into Rome July 5th cause the tickets were so cheap. $317 USD one way, I then travel to Paris and then London. But before I can book the hotels I need to know my approximate arrival date and time. I traveled to Europe for the first time this past July and I was stuck in Lille from Paris to Milan for 8 hours over night cause I booked the wrong trains