How often can I request an Unlock code from my wireless carrier?

Most GSM carriers (AT&T, T-Mobile) allow you to request one unlock code per line every 90 days (3 months). So if you have 2 phone line with T-Mobile, you can request total of 8 unlock code per year.

Of course, there are some exceptions: AT&T has exclusive contract with Apple iPhone until 2010. So they won't give you unlock code on iPhone.

Also, I've tried to get unlock code for G1 from T-Mobile, which they haven't granted after 3 weeks waiting.