Also he wasn't able to predict when he would kill Voldemort. When he did duel Voldemort I don't think he would have time to drink it. He would already be dead.
What if I told you that in the book Ron, hermione, Ginny and Neville all drank some so that while they were searching the halls for basilisk fangs and without it they would have been killed with a stray spell
Read book 6
Here is proof
On page 612 bottom paragraph
"Harry, if we hadn't had your Felix potion, I think we'd all have been killed, but everything seemed to just miss us.-"
I'm sure there's more but that's what I found at the time
What if I told you they weren't searching for basilisk fangs? Harry gave them the rest of his potion while they monitored the halls, in addition to aurors, because he and Dumbledore left to go find the horcrux in the cave. He suspected that Draco might finally carry out whatever scheme he had been plotting for months (sorry for adding on to this so late... Going through randoms and I have to correct that)
Here is proof
On page 612 bottom paragraph
"Harry, if we hadn't had your Felix potion, I think we'd all have been killed, but everything seemed to just miss us.-"
I'm sure there's more but that's what I found at the time