when do i need to replace my battery?

See this excerpt from the "Your battery's maximum capacity" section of the support article: iPhone Battery and Performance - Apple Support

"A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions. The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers battery service for a charge." In other words, if it is below 80% maximum capacity, it is below normal performance. You can still use it but you may wish to consider having it changed.

Go to this web site for information about how to request battery service for your iPhone: iPhone Battery Repair & Replacement - Apple Support

For cost and availability, enter your model information in the "Get an estimate" box at the bottom. (I will let you do this since the answer will depend upon the country where you are located.)

For instructions on how to get your iPhone, iPad, or iPod touch ready for service click here --> Get your iPhone, iPad, or iPod touch ready for service - Apple Support

Since it has reached the critical point of its life (80%), I suggest that you have it replaced, because it can have unpredictable behavior. (also create a full backup of your iphone content ASAP).

Make an appointment (*), and get an estimate here:--> Choose Your Country or Region - Apple Support

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among this list:--> https://locate.apple.com/choose-country-region
• Using the "Apple Store" app, available and downloadable from AppStore.