You can cut it anywhere you want however know that it won't grow new leaves out of the cut for a long time if at all. I found that the best place for me to cut it while I still owned some was at the branching points.I started out with 1 3" stem and it turned into a whole 10g full. Eventually I got rid of it because it didn't fit in a 10g and didn't work w/ my 25g.
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