[3A2D50]

>But nothing is known about how long the NSA keeps records.

They've admitted to at least five years.

See 1:58:45 [1] when Feinstein confirms with Alexander.

[1] http://www.c-spanvideo.org/program/CybersecurityThreats

[graue]

More precisely, at 2:00:35, "...and are deleted after a period of five years."