When did you realise the main reason Snape was never DADA teacher was because of the curse?

Dumbledore knew about the curse, or at least the very suspicious coincidence that every DADA teacher after Voldemort was rejected the role only lasted a year, and he knew that Snape was too valuable to risk. That's why Dumbledore doesn't give Snape the role until the very same year he plans for Snape to kill him.