If you flush and do not remove absolutely 100% of the flush you start off acidic as the flush is acid.
Old school green coolant that they say is good for a year is even good for as long as 4 years, I've run it that long in many cars and that was before the longlife type showed up. Zero issues doing it.
The longer life type if OAT can be worse, it does not even begin to control corrosion until it has been in the car for 6 months, and I have had corrosion issues due to that. Why I run the older stuff, instant protection right out of the bottle.
Look at the color when you first put it in and the then determine how much it changes, if it does then time to switch it. The problem I had with longlife was that it began to change color in like 3 months.