How long does Steam save your cart?

72 hours from its failed purchase time.
(edit: wrong understanding)

Happened to me last night. I thought I messed something up, had no idea it's 3 days... you learn something new every day :)

just for info..::

adding steamwallet resets your cart too... happens every time in my case...

From an e-mail I got during last year's summer sale:

There was an error with a recent Steam purchase.
We've saved your cart (including discounts) for 72 hours so you can attempt your purchase again.

So yes, 3 days!

For me, I had some things in my cart since the beginning of the sale till yesterday