How long will the i-601 waiver take in south east asian country?

Bangkok is a regional USCIS office, so they keep pretty busy. two months isn't unusual at all. if you have an Alien file in the US based on your overstay, they'll want to see that before making a decision, and that could slow things down. But they don't have to make the decision in any particular period of time. it's more a matter of how busy they are.