It took me about 4 days. Make sure that you are sticking strictly to the rules for induction and not eating <b>anything</b> that isn't on that list of allowed foods.
Also, make sure you are eating as much fat as possible, 70% of your diet should be from fat (I know, sounds weird, doesn't it?)
Make sure your carbohydrate intake is less that 20 grams a day.
Check for hidden sugars, flour or cornstarch in foods that you eat. Sometimes we have to read the food labels very, very carefully to catch the sugar. Even a minute amount could keep you from ketosis.
Some people never see a change in the ketostick but that doesn't mean that they aren't in ketosis. It just means they aren't throwing off enough ketones to register on the test. That's the Atkins books says anyway.
Are you sticking strictly to Atkins or have you modified it in some way?