Learn English – “by the time” vs. “when” in a particular sentence

As is used to mean because, but it is also used when two events happen at the same time.

In "I must stop now as I have to go out." it means because, but in "She watched him as the train passed close to his house." it doesn't mean because.

As for the sentences you used as examples, both are correct.

The first assumes that a time will definitely come when the speaker’s need for the object comes to an end. The second makes no such assumption.