Treat it as plural. She was gone one and a half hours.

In your example, the phrases "one and a half" "two and a half", and so forth, are followed by a plural noun. "We were there for one and a half days." Half is a measure, an amount, a quantity. Half can be used alone with plural nouns and verbs. "Only half of the citizens vote." Half can be used with an uncountable noun and a singular verb. "Half of the food was wasted."

The correct way would be to say it in the singular form. "She was only gone for only an hour and a half." But the moment it goes to 2 hours or more than you would use the plural form. For Example: "She was only gone for two hours." In this case you would use the plural form. Hope this explanation helps. If you need help in the future please don't hesitate to reach out.