There's no such thing as a hard-and-fast rule. If there's an obvious checkpoint, use it. If there are no obvious check-points for a long way, look for a less obvious one. On a 500nm leg, one check-point isn't enough. On a 10nm leg, you probably don't need any checkpoints excect your turning point or destination.
If I find myself uncertain of my position, I'll often look at the chart in-flight for potential checkpoints coming up in the next few minutes to confirm I really am where I think I am. On the other hand, if I'm following an obviously line feature such as a motorway, with towns at regular intervals to check my progress, I probably won't plan any checkpoints at all, and if I want an update on my position at any time I can just look out of the window.
FFF
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