A vague answer for this will be, when you feel your AC having some problems with it or when it has gotten old more than 10-15 years. A much more in-depth answer would be if you start facing issues like frequent refrigerant refills, it is not serving your cooling needs; the maintenance price is getting ridiculously higher. These few tell tails should be your warning. It is always ideal to make sure to start considering your options before the device gives in and then you have to make a quick and rushed decision of finding a replacement.