I have some Arum Lilies that are getting pretty big. Is there some sort of general consensus on when a plant should be moved to a bigger pot. I would like for them to get as big as possible.
We had first better establish which plant you mean - Arum lily applies both to Arum (a bog plant usually grown outdoors) and Zantedeschia. If this is a houseplant, I'll assume it's Zantedeschia of some variety - these are rhizomatomous, and the usual time to divide these will be July/August. In terms of simply potting on to a larger pot, this can be done in Spring if the plant has outgrown its current container.