I would think there would be some kind of guideline,contact a Toyota dealer.
I think if the motor keeps running when stopped, might be an indication that battery is wearing down, and if mpg is decreasing. Might try Googling and see if there's any way you can figure out via your display.
I thought I'd heard it was at least 10 plus year battery life.
If non of your warning light are on, you are good. When you notice drop in fuel economy, or if your state of charge bar graph goes up and down quicker then before, most likely your battery is on the way out. You don't have to buy a new battery from a dealer. ExclusivelyHybrid.com has a few options for you. You can contact us at 561 299 0609. We have completely refurbished and NEW cell batteries for your vehicle.
Looking for a Used Prius in your area?
CarGurus has 207 nationwide Prius listings starting at $3,995.
Search Toyota Prius Questions
Toyota Prius Experts
Related Models For Sale
Used Cars for Sale