There is a way to measure wear on the cassette by measuring distance between teeth, if you know what it's supposed to be. There's an extremely technical YouTube channel about this.

But I just replace the chain when it's worn (as per the chain checker tool) and then see if the new chain skips. If it does I change the cassette.

As for how often to do things just keep an eye on everything. Make sure wheels spin nicely, crank has no play, and headset isn't clonking. Replace brake pads when they look worn.

Deep clean the chain as often as you can be bothered. It doesn't do it any harm. It might last longer and be more efficient but mainly clean it because nothing beats that clean chain feeling!