International data roaming

I know that I have a certain amount of free high-speed data available while roaming in other countries.  Once that is exhausted, does it automatically slow down to the free 256kbpm rate, or do I need to manually restrict the phone from using the faster rate to prevent myself from being charged for the higher speed usage?  I am hoping it does it automatically to it remains free but I’m not sure how it works and I don’t want to be charged extra.