In spoken Spanish especially, the indefinite article (un/una) is often omitted when the unit of time is one and a half, e.g. hora y media.
-Estuvimos mes y medio en España— We were in Spain for a month and a half
-Tardé día y medio en hacerlo — It took me a day and a half to do it
-Tras hora y media de espera, me fui — After an hour and a half wait, I left
-Lo hice hace semana y media — I did it a week and a half ago