Effective.
If you take the pill 100% correctly it is 99% effective. You are 99% protected the whole time you are on the pill. Throughout your whole cycle, as long as you take the pill properly.
As for when it is effective, just look up the yasmin website and see. My doctor told me three months until it is the complete 99% effective, so that is what I go by, but most of the pills website say a week.
But, if you do not want a baby, then use condoms also. Or foam, diaphram, spermicide or something.
If you have more than one partner, or do not know for sure that your partner does not have std's then use condoms. It is far to dangerous not to.