I have a beautiful oak tree that leaves turned brown and almost over night appeared to be dead. When is it too late to use the sick tree method? This is going on two-three months now.

It is never too late. The tree may be too far gone, but you are certainly doing a good thing for the soil, for the next thing to grow there, if this tree does not survive.
Having said that, it is great to see something come back when you thought it wouldn't.

If you read this, Dianne, tell us if it woke up for Spring.