Lower Annual Percentage Rate (APR) & Building Credit?

I just started building credit and was wondering how long OR how much credit history you need to have established before a credit card company would consider lowering your APR?

Also, does it affect your credit score (when trying to establish credit history) if you cancel a credit card – balance always paid on-time – and apply for a lower interest card after having the initial card less than a year?

This is my first card at 0% interest until Dec. at which point it goes to 16.75%. It would be nice to have found a card that offered a better rate, but I had to take what I could since I had no credit history!

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