Simple, eight has a 60 days inactivity policy before your eight line gets terminated. Their paid plans lasts for 30 days.
For example, 1st January pay $8. 30 January your plan ends. 31 January you don't pay, 1st February you pay, plan ends on 2 March, 3 March - 31 March you don't pay, 1st April pay $8, rise and repeat. Just remember to buy a paid plan on the 1st day of the month, this can ensure the starting date is the very first of the calendar month. Ending date is impossible to end as the paid plan is on 30 days period.