How soon after starting the pill are they 100% effective?
Answers: they say about a month .
They are never 100% effective.
"Two ways to start taking the pill for the first time
A first day start means you start taking the pill on the first day of your menstrual period. With a first day start, the pill may be effective as early as that first day, but waiting one full cycle before going without a backup is strongly advised.
A Sunday Start is when you start the pill on the first Sunday AFTER your period begins (or, if it starts on a Sunday, on that Sunday). When you start with a Sunday start, the pill may be effective as early as within one week, but waiting one full cycle before going without a backup is strongly advised. The Sunday Start method was devised primarily for women who would prefer they have their withdrawal bleeds (your period wile using the pill) on a weekday, rather than on weekends, as it makes that more likely.
Unless your healthcare provider suggests one way of starting is better for you, how you start is your call, based on your preferences. These two ways are ONLY relevant when you first start taking the pill. For every cycle thereafter, you'll start your new pack when you finish the pack before."
http://www.scarleteen.com/article/advice....
No form of birth control is every 100% effective. After taking the pill for 7-10 days it becomes fully effective (still not 100% though). Most doctors recommend using alternative precautions for the full 28 days (first pack). Never, however after the first month of use, they are functioning as much as they are going to.