My iphone 13 battery got 99 in 5 months is it OK?

If it's only dropped 1% in five months, then you're doing great!

Batteries degrade over time with usage, that's just how they work. You can expect about a 1% drop per month with average use on an iPhone.

Apple recommends replacing it when it reaches 80%. To check your battery health, go to Settings > Battery > Battery Health.

Here are some tips on maximizing battery life.

Batteries - Maximizing Performance - Apple

99% battery health is just fine.

How long it takes to degrade is not so much time based, it's how much you use and charge the battery.

iPhone Battery and Performance - Apple Support

About the battery and performance of iPhone 11 and later

Just look at a battery replacement if it gets down to ~80%