There are no official limits; that is the GDS systems do not impose a limit. Each airline is free to release their fares and control booking based on their individual requirements.
You can usually find this information in the agent reference/manual/guidelines published by airlines. These also include the fare rules, restrictions, refund policies, penalties and a lot of TLA (three-letter acronyms) that are only relevant to those in the industry.
You simply have to search for the airline you are interested in, here are some examples that I found:
Frontier has it available on their page for travel agents, written in plain English (my emphasis):
Frontier participates in the following Global Distribution Systems:
Amadeus, Sabre and Travelport (including Apollo, Galileo and
Worldspan). Please be familiar with the specific formats and
requirements of each system for booking and ticketless/Guaranteed
Frontier utilizes teletype (TTY) connectivity for availability and
will open and close inventory with AVS messaging. F9 flight schedules
will be available for booking up to 180 days in advance of departure
date in the GDS systems.
American Airlines, states on their website:
You can book a flight on aa.com up to 331 days in advance.
British Airways publishes fares one year in advance:
You can access information up to one year in advance.
Finally, a route is different than a flight. A route is a combination of arrival and destination (with optional stops), but a flight is a timed departure for that destination. There are some routes that are seasonal, some routes that are permanent - and the availability of flights also fluctuates.
There are usually multiple flights on a route. An airline may have multiple flights on the same route; and multiple airlines may offer different flights, or even flights departing at the same time (often within 30 minute gaps). This is often the case on popular/busy city combinations.