Any time ;)

Just remember that if it ends with -us, it probably pluralizes to -i. That's only for latin-based words. Greek-based words, like octopus, can either be pluralized to octopuses or octopodes (pronounced Ahk-top-o-dees). And sometimes you have a new or technical latin-based word like "virus" which just pluralizes to "viruses." It's perfectly fine to pluralize uterus to uteruses, too, since it's so uncommon. English is a bitch.

[Edited to give a longer explanation]