Check your battery health, free storage, charging port, and update support, then decide what would actually force you to replace your current phone. A calendar deadline is less useful than a trigger such as unreliable charging, random shutdowns, or no longer getting through your usual day.
I would not wait for the iPhone Duo to be “proven,” because that standard could keep moving for years. Instead, wait until the first wave of boring owner feedback replaces the launch noise. People posting after daily commuting, pocket lint, cold weather, beach trips, and thousands of folds will tell you more than somebody bending a display under studio lights. You do not need perfect failure-rate data to notice when many unrelated owners start describing the same oddly specific problem.
There is another cost to waiting until your current phone completely dies: you may lose its trade-in value and end up shopping under pressure. That does not mean buy now. It means pick a point where you will sell or trade the old phone while it is still functional. Keep a current backup and make sure photos, messages, authentication apps, and account recovery methods can survive a sudden failure. That preparation matters more than guessing the ideal purchase month.
I slightly disagree with treating a battery replacement as the automatic cheap answer. It makes sense if battery life is the only issue. If the phone has a worn port, damaged camera, unreliable buttons, or too little storage, putting more money into it may only postpone the same decision. Get an actual estimate first and compare it with how much longer you realistically want to use the device.
If you do buy the Duo early, use the return period to test the ordinary annoyances rather than trying to recreate a torture test. Check the crease outdoors, type for a while, use it one-handed, carry it in your normal pocket, and see whether opening it constantly gets old. Those things can make you regret the purchase even if the screen never fails.
My cutoff would be practical rather than time-based: keep the old phone while it remains predictable, watch for a repeated failure pattern on the Duo, and replace before the old one becomes an emergency. Waiting a few months lowers the odds that you become unpaid quality control, but there is no month when a folding screen suddenly becomes a sure bet.