how come sometimes my lentils take an hour to cook vs 20 min.

Another factor is the hardness of your water -- similar to the salt issue, I suppose. Try filtered water instead of tap as an experiment.

Indeed it does depend very much on the type of lentil from those big coin like ones to the little green lentil de puy. Also your altitude can have an effect as well as does the age of the lentil. If they are old they will take longer. But 20 minutes is kind of on the optimistic side for any dried legume. I figure more like 40 minutes.

Depends upon the type of lentils.. the split dehusked ones get done much faster. . Also, if you salt the water, it will take longer..