Philosopher's Stone has a lot to say on this subject.
You have slain something pure and defenceless to save yourself and you will have but a half life, a cursed life, from the moment the blood touches your lips.’
Philosopher's Stone - page 188 - UK - chapter 15, The Forbidden Forest
So each time the unicorn's blood touched Quirrell's lips, the curse of a half life, a cursed life, would be enacted. My impression is that it only takes one time drinking unicorn blood to invoke a life-long curse, unlike a spell which wears off after a given period of time. I would imagine the curse is lifted at the time of the cursed person's death. No, the curse from drinking unicorn blood does not cause the head of the person possessing you to grow out of the back of your own head -- remember, Voldemort possessed Quirrell's body all the way back in September, maybe even August, of 1991 (HP LEXICON - TIMELINE).
Harry spotted Professor Quirrell, too, the nervous young man from the Leaky Cauldron.
He was looking very peculiar in a large purple turban.
Philosopher's Stone - page 91 - UK - chapter 7, The Sorting Hat
As far as a timeline for drinking the unicorn blood goes, during Harry's first year (1991-1992) Harry, Hermione, Neville, and Draco serve detention with Hagrid on or around May 26, 1992. Harry and Draco came across the carcass of a unicorn in the Forbidden Forest, and then encountered Voldemort (in Quirrell's body) drinking the unicorn's blood [HP LEXICON - TIMELINE].
The cloaked figure reached the unicorn, it lowered its head over the wound in the animal’s side, and began to drink its blood.
Malfoy let out a terrible scream and bolted¹ – so did Fang. The hooded figure raised its head and looked right at Harry – unicorn blood was dribbling down its front.
Philosopher's Stone - page 187 - UK - chapter 15, The Forbidden Forest
The other slain unicorn was found the Wednesday before, within a week of the Forbidden Forest escapade recounted above:
‘Look there,’ said Hagrid, ‘see that stuff shinin’ on the ground? Silvery stuff? That’s unicorn blood. There’s a unicorn in there bin hurt badly by summat. This is the second time in a week. I found one dead last Wednesday. We’re gonna try an’ find the poor thing. We might have ter put it out of its misery.’
Philosopher's Stone - page 183 - UK - chapter 15, The Forbidden Forest
There are no other instances of Voldemort drinking unicorn blood in Philosopher's Stone, although he indicates he drank it over the course of "weeks" via Quirrell and the effects were strengthening:
‘See what I have become?’ the face said. ‘Mere shadow and vapour ... I have form only when I can share another’s body ... but there have always been those willing to let me into their hearts and minds ... Unicorn blood has strengthened me, these past weeks ... you saw faithful Quirrell drinking it for me in the Forest ...'
Philosopher's Stone - page 213 - UK - chapter 17, The Man With Two Faces
Your last question about Quirrell being equally as cursed as Voldemort is a great one, because according to J.K. Rowling, Quirrell was completely subjugated by Voldemort, and although he attempted at times to resist Voldemort, Voldemort was too powerful and always retained control.
That said, Quirrell isn't innocent. See Pottermore info on Quirrell [HERE] and [HERE]. Quirrell sought Voldemort out with a naive arrogance, not understanding how dangerous Voldemort was, even in spectral form.
So was Quirrell cursed as well as Voldemort? He certainly didn't live much longer after he drank the unicorn blood.
ETA: Regarding how the potion with the unicorn blood reacted when Voldemort took it, I cannot give a definitive answer. We know that drinking unicorn blood will give a person "a cursed life, half a life" but we don't know if brewing unicorn blood (heating it) or blending it with snake venom changes its composition enough to bypass the curse. There's just not any canon information about this; it's not addressed. I could speculate, but it wouldn't be canon.
Regarding Voldemort's face on the back of Quirrell's head, there's nothing to indicate that unicorn blood had anything to do with this. Voldemort had possessed Quirrell at least by September 1, 1991, and Quirrell was wearing his turban to hide Voldemort's face by that date. The earliest we know for sure that Voldemort was drinking unicorn blood is on or around May 19, 1992, a full 8.5 months later. And because Quirrell was killed, and Voldemort abandoned Quirrell's body as a host, we don't know if the face would've lasted infinitely.
As for the "half a life" issue, it's either literal or metaphorical. If it's literal, then Voldemort should have lived for an additional 70 or so years past the date that he actually died (May 2, 1998), for he was approximately 70 when he died. This would be without Horcruxes, of course. If it's metaphorical, then it suggests Voldemort's life, as Rob said in his comment, wouldn't be fulfilling. Kind of like A life lived in fear is a life half lived².
²From the movie Strictly Ballroom