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