I see a service notification against my MacBook Air (early 2015) battery icon. How long does the battery last typically before replacement?

Unfortunately beginning with macOS 10.15+ Apple has changed the reporting of the battery condition making it impossible to properly interpret the true condition of the battery since a battery with a capacity less than 80% of the original design capacity will show this "Service Recommended" condition as well as when macOS detects an actual hardware fault with the battery.

Run the Apple Diagnostics to see if it reports a battery failure. If the diagnostic does not report a battery failure, then the "Service Recommended" condition means the battery capacity is below 80% of its original design capacity. You can use an app like Coconut Battery to check on the actual battery charge condition in relation to the original design capacity. As long as the diagnostics do not report a battery hardware issue you can ignore this message until the battery no longer performs adequately for you.

  State of Charge (%): 77

That is less than 80%

Quote.

Service Recommended: The battery's ability to hold charge is less than when it was new or it isn't

functioning normally. You can safely continue to use your Mac but you should take it to an Apple Store or

Apple-authorized service provider to get your battery evaluated.

https://support.apple.com/en-us/HT204054

In my experience, I had to replace battery after about 200 cycles and 5 years.

I use my MBP in place of a desktop.

Use it as long as it works if it doesn't affect the performance.

Regardless of Full Battery Cycles - battery are Consumable Product and as such degrade over time. Usually, when the Service Battery Notification appears - it is an indication the battery does not supply reliable power to the computer. Yes, you might still be able to use it in this state but at some time the computer may start to automatically shutdown and thus any work in progress will be lost. Better to consider getting it service only by Apple Authorized Service Centres.

Hard to say. That depends on usage pattern.

Please read this article for info. https://support.apple.com/en-us/HT201585