Easiest way to know you need a battery replacement: Your phone gets to 40% or 20% and suddenly just dies on you. That means at least one cell in your battery is dead, and it's time to replace the whole battery.
Also, if your screen is edging up on the left side but isn't cracked or broken, that's a sign of an exploded or "puffy" battery. Usually people have the first issue for a while, don't realize what's happening, and then eventually it goes puffy and explodes. It can even break the screen when it does that.
As with other repairs, if you get a battery replacement at a repair shop, ask about the warranty. Replacement batteries can have real issues, and some runs of them have high defect rates. No big deal if the shop you work with has a good warranty and will just swap it out if it stops working.
Since I was out of warranty anyway I opted to self-replace the battery.
Wait, why cant the OS tell the user that something is wrong with the battery.
That's been the only "killer" for phones for me. With a decent case I feel like they otherwise last forever.
My personal upgrade cycle usually involves someone else's (ex: spouse or parent) phone breaking, giving them mine, and buying a new one. Otherwise I'd probably still be on an iPhone 4.