Start with the install year. It is the one fact that turns a system into a planning date. You can usually find it on a receipt, a permit, a service tag, or the data plate on the unit itself. Find it before you do anything else, and write it down next to the room or duty the system serves.
Turn the install year into a window
Open the guide for that system and read its typical service life. That range is a planning input, not a forecast for your unit. It shows when the decision day may come into view, so you can start setting money aside while the system still works well. Treat the window as a span of years, and check it against the system's condition at each inspection.
Set the reserve as a standing line
Keep a separate line in your household budget for each major system. Put a fixed amount into it on a regular schedule, and label it with the system and its window. The goal is not to guess a price. The goal is to have cash ready so the decision day does not force a rushed choice.
Revisit the line each time you inspect the system. If the unit has needed more repair visits, move your window closer in your notes. If the unit has been lightly used and kept up, leave the window where it is. Do not write a replacement date in stone.
What to check before the decision day
When the window opens, ask licensed trades for written quotes. Compare what each quote includes, what it leaves out, and whether it matches the system size and location you recorded. Keep the reserve on hand until you have the written scope in front of you.
If you carry a service contract on the system, read its renewal, notice, and cancellation terms before the plan rolls over. Those terms can change how much of the reserve you need to hold back. The published tables on this site list typical service life for each system, and the system guides explain the planning window in more detail. Start with the Central Air Conditioner guide if that is your largest cooling cost ahead.