How low should I let my MacBook Air battery go before charging?

Leaving it plugged in all the time is fine. The charging circuits and OS will manage the battery.

The best advice for getting the most from your battery:

• Plug in when using the computer near available power.

• Use on battery when ac is unavailable.

• Don’t subject the computer and battery to extreme temperatures.

• Let the OS and charging circuits manage the charging.

• Try not to allow the battery to discharge deeply regularly before plugging in. Yes, it may happen from time to time, just don't make it a habit.

• Don't obsess over battery life and/or health.  

Here is Apple's guidance on the subject.

Batteries - Maximizing Performance - Apple Support

About battery health management in Mac laptops - Apple Support 

❝Worry is the misuse of imagination.❞ – Zadra

"How low should I let my MacBook Air battery go before charging?: is it ok to leave MacBook Air i3 plugged in all the time, or how low should I let battery go before charging?"

-------

Use a Surge Protector:

You should most certain leave it plugged in all the time, but do su by plugging it into a surge protector, to protect it from power surges and power spurs. If a lightning both or tree where to fall on a transformer down the screen, then power may go into the curets, ultimately, rerunning in device power damage.

+1

You're welcome.