If you have a prepaid mobile, the month is calculated on a calendar basis. For example, if you reach 300GB of usage on May 28th, you will continue to use the internet at 4Mbps until May 31st, and on June 1st your speed will return to full speed. Similarly, for every month of the year, keep in mind that your speed will reset on the 1st day of each month.
If you have an internet4share connection, then the month is calculated based on your billing cycle. That is, if your contract benefits renew every month on the 15th, then on that same day (the 15th), your mobile internet speed will also return to normal levels.