When I first passed the PPL I was tending to use checkpoints much too close together for fear of getting lost. This only resulted in too high a workload which could have resulted in me getting lost!
I now tend to use checkpoints at around 20nm intervals, but it all depends on the route. Having drawn the route on the chart I look for easy to spot features (such as a lake with a railway line running alongside it) either on-track or very close to track, and then mark off points along the route with numbers and put the corresponding numbers on the PLOG. Sometimes the checkpoints may only be 10nm apart, sometimes 30nm. The important thing is to be able to positively identify a landmark rather than confusing one lake for another, or one town for another etc.