Most collectors agree the first issue of T206s were issued in mid-season 1909 and they were marketed at least through the early part of the 1911 season. In fact, evidences shown T206s came from stock pictures produced earlier than 1909 and there's a chance they may probably issued before 1909.

Evidence #1:
T206 inclusion letter to Neal Ball
On the letter dated January 29, 1909 written by newspaper writer, Bozeman Bulger, to Neal Ball asking for Ball's permission to be included in the T206 set. Bulger states in his letter that he already has the photos on hand for use in the T206 set. From this letter, it provides strong evidence the photos used for the 1909 edition of the T206 cards came from stock pictures earlier than 1909.

Evidence #2:
Mystery CINCINNATI Reds "Ghost" on T206 misprint
On that T206 Casey card, there is a red image of another player printed upside-down on the front and that secondary image is not any players from T206. From the secondary image, it is clear the mystery player was played for Cincinnati Reds. Emblazoned on the chest of that image is a bold, gothic, curved "CINCINNATI". According to the "bible" of Baseball Uniforms of the 20th Century, this particular logo type was only used on the Reds road jerseys for a single season - 1908.

From these two evidences, there is no doubt that T206s came from stock pictures and prepared way before 1909. It is also possible they might even issued before 1909.