I think it's up to one month or $200 which ever comes first :)

I roughly worked out, that using all 20 cores you will get about 120 hours for free, my math may be out slightly though?

I'm using this as well. You get 20 free cores for 1 month. I get about 210 coll/min from all the cores. You'll need a credit card to setup the free trial.

Just thought I'd post this as I've just started using it and it's been ok so far.

$200 free trial up to 20 cores :)

http://www.windowsazure.com/en-us/