+1 for this.
It's also something you have to learn and judge for yourself.

In times when funds were short, I'd wait until the chain (adjusted as best as it would go) started falling off the rear sprocket on corners. - Can't do that too often in traffic!

Or even earlier times, just wait until it snapped - - go running back down the road to retrieve it and use a couple of spring links to put it back on. But with modern chains though (much heavier than years ago) and lighter-weight modern engine casings, that's a recipe for disaster nowadays.

To save a considerable amount money, make sure it's properly lubricated all the time. Either manually, or Scotoiler or LubeMan, they all work pretty well with ordinary engine oil (or Scotoil in their case).
In my experience, I have learned to avoid the aerosol spray can chain lubricators that squirt out a gooey mess. They attract the dirt and grit which then becomes pretty difficult to wash off and still continues to wear everything away. Using engine oil helps the grit to fly off, and leaves everything a lot easier to clean.

Lastly: You will find you will use a lot more chains than anything else.
I've found that depends on the bike, and whether you use manufacturer's genuine sprockets as replacements. Sometimes they're prohibitively expensive.
Usually I've replaced chain and sprockets together. Sometimes the sprockets are OK, just change the chain.
On the Yamaha I now have, the sprockets wear faster than the chain.
Different engines (singles v multi-cylinders) and different hubs (different cush-drives) wear the transmission in different ways.

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TTR250 - London to Cape Town