Snow, in Portugal means only Serra da Estrela, and only for short periods.
That's why the biggest winter sports resort for the Portuguese stays in Spain, exactly in Sierra Nevada. But if you are no winter sport lover, then spring is the best time to visit it, while there is enough snow to compose the look of the place, and, down the mountain, Granada starts to suggest the oven that Andaluzia becomes in summer.
The interest of the place is snow. So, if you don't intent to practice, a couple oh hours is enough to wander around, make your pictures, and go down to Granada, where there is lots to do and see. If you want to do it, you better consult a specialized link like Sierranevada or Cetursa