Absolute bollocks on letting it run down on a regular basis, it will be a Li-Ion battery and running them flat is what speeds up their degradation.
Modern laptop batteries have built-in electronics that will be monitoring the battery health and trigger the alert once it starts to degrade past a certain point (probably it’s rated half-life).
Most OEMs warranty a laptop battery for 12-18 months but they’re usually good for 3-5 years but it does depend on use (again, if you regularly fully discharge it you’ll shorten this time). It should just gradually degrade though so just replace it when it stops holding enough charge to be convenient for the way you use the laptop. And don’t buy a cheap Chinese import battery from eBay, you might get lucky but chances are you’ll get a dodgy quality one that’s probably a fire risk.