I know you must be in Arizona for 90 days to get divorced. Is that true of legal separation?

No. For legal separation, one of the spouses just needs to be domiciled in Arizona. Domiciled means you live here, and you intend to remain living here.

But to divide up property, Arizona will need to have personal jurisdiction over the other spouse (Arizona has it if the other spouse lives here or has a significant connection to the state; if you are unsure about if Arizona has personal jurisdiction, consult with an attorney). If children are involved, the home state must make those decisions. The home state is either the state that issued the current custody order or the state where the child most recently lived for six consecutive months.