what you are asking about is ovulation... not conception.
I can't tell you when because its not a definite number and the length of the follicular phase (the time from the first day of your bleed to the day you ovulate) can vary dependant on stress, illness, medication etc.
It is the luteal phase that doesn't change.
I would strongly recommend you read Toni Weshler's "Taking Charge of your Fertility".
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