How long does it take the average couple to conceive?

im asking this because last night i talked to my dad and stepmom about them maybe reconsidering the idea of having a baby ((hes 46 and shes 31)). until now, they never knew that I wanted them to have a baby, so they told me they may reconsider. if they start trying for a baby soon, how long would it be until my stepmom becomes pregnant?