Nobody knows for sure, but it is supposed to last until Portal 2 comes out on April 19th. On the Potato Sack store page it says, "Hurry, this special offer ends when Portal 2 releases."
http://store.steampowered.com/sub/7586
"Hurry, this special offer ends when Portal 2 releases."
I wonder if this still applies if the game releases early. either way the potato pack has been great for the indie developers involved.
Portal 2 comes out on April 19th. On the Potato Sack store page it says, "Hurry, this special offer ends when Portal 2 releases."