I started with AdSense in January of 2004 and was over $1,000/mo by March of 2004.
But... I started with one website which was receiving 200 UVs/day and another which was receiving 100 UVs/day. That gave me two nice platforms to build form.
Before I started with AdSense, these were just hobby sites. When I realized how profitable AdSense could be, it immediately grabbed by focus.