Sparrow nest box

Hallo. My bird-corner is in a balcony. Almost 12 years ago, I put a nest box on the most repaired wall. Nobody ever visited it and I was starting to consider it as a relic. But today I saw a straw hanging from the hole so I started observing it attentively. A female sparrow is building her nest inside of it, she is very much busy. I see she is a young female, maybe at her first experience. Now, I read that sparrows can have up to 4 broods per season. Moreover, I also read that during winter the couple still uses the nest as a night shelter, while the youngsters sleep on trees. At the same time I read that I should clean the nest box when the nestlings have left it definitely. The question is: shall I clean it after each brood or at the end of the season, say in late Autumn? And, if and when I have cleaned it, will the sparrow couple still use it in winter, without any straw and so on inside of it? I forgot to specify that I am in South Sardinia, so I am talking about Sardinian Sparrows (Passer hispaniolensis). Any suggestion? Thanks.