frankly, very easy to expire...
eg1: u made a good calculation for it, but hotlink change the "validity" rule for your "plan" without inform in advanced, so ur number expire and u stuck ..(ya, especially validity365 and validity90)
eg2: u ask your family or friend take care of it for u, they doing it months by months but one, then ur number expire and u stuck..
eg3: so on....