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 

Few trains can be booked a year in advance. French and Italian trains can't. Additionally, trains are not always opened for reservations at the same time and then a journey planner may propose strange routes, such as Paris to Milan via Lille.

As ​@rvdborgt said, you won’t be able to lock most trains in a year in advance. This page gives a good overview of when reservations open.

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

If you are booking so far in advance, locking everything in, and not making use of the flexibility of a pass, you’ll probably find that point to point tickets will be cheaper and easier than using a pass - especially for Italy, France and London. 

If I was you I would plan out where I want to be (which you have done) and when I want to be there.  Then book hotels.  Then book trains.

Look at it this way - Rome to Paris will mean going Rome to Milan as a first step and then connecting to Paris.  Looking at things today there are 2 Trenitalia trains and 2 Italo trains every hour between Rome and Milan.  You will not be stuck for choice.

Then from Milan to Paris there are 5 trains per day.  Again, you will not be stuck for choice.  The timetable may vary by a few minutes here or there, or by an extra train or one less, but it’s not like the entire route is going to be cancelled.

Paris to London - there are something like 10 or more departures a day.

The fact of the matter is that because of the way some timetables are released you may well have to wait until some time in June to be 100% sure of the exact timing, but you can rest assured that for the routes you called out you will have plenty of options no matter which days you pick.