1) First of all, it's very debatable whether stimulus bills work at all. As Walter Williams has pointed out,
"In stimulus package language, if Congress taxes to hand out money, one person is stimulated at the expense of another, who pays the tax and is unstimulated. A visual representation of the stimulus package is: Imagine you see a person at work taking buckets of water from the deep end of a swimming pool and dumping them into the shallow end in an attempt to make it deeper. You would deem him stupid. That scenario is equivalent to what Congress and the new President proposes for the economy." -- Walter Williams
2) According to the Wall Street Journal, there is very little actual "stimulus" in the stimulus bill,
"In selling the plan, President Obama has said this bill will make 'dramatic investments to revive our flagging economy.' Well, you be the judge. Some $30 billion, or less than 5% of the spending in the bill, is for fixing bridges or other highway projects. There's another $40 billion for broadband and electric grid development, airports and clean water projects that are arguably worthwhile priorities.
Add the roughly $20 billion for business tax cuts, and by our estimate only $90 billion out of $825 billion, or about 12 cents of every $1, is for something that can plausibly be considered a growth stimulus."
3) Even if you set aside #1 and #2, a significant percentage of the spending in the stimulus bill doesn't occur for two years.
"Although the argument for quickly passing the $800-billion-plus stimulus bill now before Congress is that pumping money into the economy immediately will spur economic growth and create jobs, almost half of the new spending—as opposed to tax cuts—being proposed in the bill would not be spent until at least two years from now, according to an analysis published by the Congressional Budget Office."