The ACs numbers make no sense, the length of the buffer is irrelevent, what matters is the rate at which you are writing date. 250 megabytes per second is also a crazy datarate, typical broadcast HD is more like 1 megabyte per second (8 megabits per second). Maybe a bit more if you have a cable provider who have more bandwidth than they know what to do with.
1 megabyte per second * 2 tuners = 2 megabytes per second = 7.2 gigabytes per hour ~= 170 gigabytes per day ~= 63 terrabytes per year
The shortest life drive in TFA started showing issues at 300 terrabytes and crapped out after 700 terabytes. So we are talking a probable lifetime of years.