Once started, payments must continue for the longer of five payment years (from the date of the first distribution) or until age 59-1/2. Once you've met the five-year AND age 59-1/2 point, you can change the distribution pattern, even stopping distributions altogether until you reach 70-1/2 when the Minimum Distribution Rules apply. For purposes of the 5-Year rule, the clock starts on the date the first distribution occurs. Also note that for purposes of the 5-Year rule you must take one annual distribution in each of the 1st 5 years for a total of 5 annual distributions.
The 'First Payment Modification Date' is calculated as one day following the later of the end of the 5-year period, or the date you attain age 59.5 - this would be the first possible date to modify (increase, decrease, stop, etc.) the planned SEPP payments without busting the plan.