How long your cycling computer's battery lasts will dictate how long you can enjoy being out on your bike, especially if you're relying on it for navigation or training data.
Generally, the best cycling computers will have a battery life of around 15-20 hours when used conservatively, but this will be reduced if you're pairing to numerous sensors or following navigation for hours at a time. Riding with the screen light on full for hours on end will also sap battery life. Not too many of us will be riding for 15 hours straight, so a shorter battery life probably won't cause any issues whilst riding, but it will mean you need to remember to charge it more frequently.
So to answer the question, look for enough battery life to comfortably cover the duration of your longest rides, then consider the time between charging, if ever you find yourself riding multi-day trips without a power source.