Don't know how long it is free to play, but I found it today, when I was searching around for a new lunchbreak game
Q. How much of the game can I play without paying anything?
The entire game can be played without making a purchase. All game modes, classes, and maps are available. Nearly every weapon is available through achievements, drops, or crafting.
Seems like there are free accounts and premium accounts. All you need is steam.
The difference is explained here: TF2 Free2Play FAQ
I am downloading now.