I've asked my GP for prescription Retin-A but I forgot to ask how much to apply. I want to get rid of whiteheads and fine wrinkles and I read that Retin-A is one of the few anti-wrinkle treatments that really work. I've applied it all over my face and looked like a scaly lizard the next day... how much do I use and how quickly does it work?

What formulation do you have?

Here is info for gel: Retin-A Gel facts and comparsions at Drugs.com
And for cream: Retin-A Cream facts and comparsions at Drugs.com

There is strength (0.025%/0.5% I think) and type (gel/creme). You can't over apply the gel.
The creame can easily do what you described.

I have used Retin A with great results. I applied a small amount to the areas that I wanted treatment on a rubbed it in until it was absorbed.

I used it for acne and it made the acne worse before it got better. All together it took about 3 months for my skin to clear up. It was totally worth it though.

Also be sure to wear sunscreen as Retin A an make your skin really sensitive to the sun and be sure to avoid harsh facial cleansers. My derm suggested anything that has a balanced Ph.