It is as important to leave a good leaf number on lilies as it is on any other bulb. We want them to gather energy with those leaves to feed the bulb for next year's bloom.
So the rule of thumb is to leave as many leaves on the plant as you can - leaves that are healthy and growing well.
Some gardeners cut the lilies back just a tad shorter than surrounding foliage so that the stalk is hidden and this is fine as long as you leave a significant number of active leaves.
The problem with saying cut it back to XX is that sometimes the lower leaves are eaten or wilting away from a lack of sunshine and aren't worth anything to the bulb. You want as many leaves as possible and that's the rule of thumb.
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