In the normal home sale, a CMA is probably enough to let you set a proper price. When real estate is booming and prices are rising, houses may sell in a few days. Conversely, when the market slows down, average DOM can run into many months. In a seller's market, you can price a bit beyond what you really expect, just to see what the reaction will be. In a buyer's market, if you really need to sell promptly, offer an attractive bargain price.
Some sellers list at the rock-bottom price they'd really take, because they hate bargaining.
If there haven't been many prospects viewing your home after three weeks, you may need to lower your list price. If that doesn't bring any prospective buyers, you may need to lower your list price again. Plan on doing that regularly until you find a level that attracts buyers. Make a written schedule in advance, before emotion takes over and you're tempted to dig your heels in. This can be particularly useful as you start looking for another home to buy.