We have already seen desde que as a temporal conjunction, see 19.2. Desde que as a conditional conjunction belongs therefore to the many conjunctions whose components suggests a meaning they actually doesn't have. As a temporal conjunction it suggests what it means, dede means since. Used as conditional conjunction the meaning is if / provided. As a temporal conjunction it requires the indicativo, as a conditional conjunction the conjuntivo.
desde que if
Tudo o que quiser, desde que estude e passe de ano.
Whatever you want provided that you pass the year.
Você pode ir brincar na rua, desde que faça todo o dever.
You can go to play in the street, when you have done all your homework.
At first glance it seems strange that a temporal clause can become a conditional clause. However perhaps this can be explained. (Although the explanation is not very convincing, but at least it is a possibility. Desde que as a conditional conjunction is speciality of Portuguese. It doesn't work in other roman languages. ) Let's have a look on this sentence.
Desde que trabalhe é mais feliz.
Since he is working, he is more happy.
We can deduce from this assertion that he is more happy, when he has a job. If this is the assertion, we can substitute this sentences with a temporal clause.
Se trabalhar é mais feliz.
If he works, he is more happy.
It is possible that this case has been generalized in the course of history. Similar phenomenons can be observed with other temporal conjunctions, for instance quando, see 19.2. Quando is never mentioned as a conditional conjunction, the difference between a temporal conjunction and a conditional conjunction can be however very subtle: 1) When he comes, I ask him. <=> 2) If he comes, I ask him. We suppose that in 1) it is granted that he comes, but actually that is not sure.
Desde que is rarely used as a conditional conjunction, although mentioned sometimes in grammar books as a conditional conjunction. Normally we use se in this context followed by caso.