“A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions.” Source: Apple https://support.apple.com/en-us/HT208387

The phone will give you a suggestion on when to do this.

Goto Settings->Battery->Battery Health & Charging

There is a line Maximum Capacity - if it goes below a certain level (70% I think) that page will also show an indicator to say the phone needs servicing.

From Apple the screen will look like

Hi Arvind, · You will have to look up some YouTube videos on how to transfer the BMS, or Battery Management System from the original battery to a new replacement cell. You could disassemble the replacement you already put in for that purpose, but vendors are now selling bare cells without the BMS specifically for this purpose so you could just buy one and transfer your BMS to that. Doing it that way means you may get the tape and longer leads that will make your replacement look more professional and probably be more secure. Here's an example of a guy who seems to know what he's doing. · NO MORE IMPORTANT BATTERY WARNINGS! iPhone XR Battery and BMS swap - YouTube · Basically the video is going to show you how to disassemble the old battery and remove the BMS. Then you need to attach it to the replacement. Using a spot welder is recommended, but judicious and minimal use of a soldering iron can also be successful. You then tape it all back up and you're ready to install it into your phone. · The one part we haven't addressed yet is the battery health. Yes, you will get it back and get rid of the non-genuine warning, but be aware that it's going to show the numbers from the old battery; it doesn't reset itself to 100% and 0 cycles automatically. Rather, you need a device programmer like the JC V1SE or QianLi iCopy. · I have both the iCopy programmer and a spot welder for just such a case, and you can figure on paying about $85 USD for the programmer and another $40 or so for the spot welder, so if you wanted to do it all yourself the right way, you can assume you're going to have to put out about $125 in a one time purchase. Of course, once you have them any future replacements are going to be a lot cheaper to do. · So if you have a soldering iron, you can get away with just swapping over the BMS to the existing replacement battery and only be out the cost of a new sheet of screen adhesive. Otherwise figure about $40 for a spot welder if you want to do it the safest way, plus a few more dollars for a bare cell; those are going to run you about $10 or so. Here's an example. · XCAP - Battery Cell without Flex for iPhone XS Max (BMS) (3710 mAh) (Spot Weld Required) - Injured Gadgets · Either way, once you have the original BMS working on a new cell, then you have to decide if it's worth $85 for you to be able to clear the health information. If you have a repair shop local to you, you could try contacting them to see if they would clear the health information for you; I have no idea how much them might charge you, but I'm pretty sure it would be less than the cost of an entire device programmer. · Good luck; let us know what you decide and how it all turns out!