Usage of "when" As an adverb. At what time: When will we leave? As a conjunction: 1. At the time that: in the spring, when the snow melts. 2. As soon as: I'll call you when I get there. 3. Whenever: When the wind blows, all the doors rattle. 4. During the time at which; while: when I was young, I was sick all the time. 5. Whereas; although: She stopped short when she ought to have continued. 6. Considering that; if: How can he get good grades when he won't study? As a pronoun: What or which time: Since when has this been going on? As a noun: The time or date: Have they decided the where and when?

Several years had passed before I finally realized the true value of friendship makes sense, whereas if you use "when" here as a conjunction then it doesn't make sense.