Bereishit 25:21 tells us that Yitzchak prayed for Rivka because she was barren and God accepted his prayer. They were married for 20 years before she had the twins. I am assuming that they didn't know on day one of their marriage that she was barren. When did they figure out that prayer was called for and when did he make this prayer?
I understand that Yitzchak is credited with beginning the mincha prayer, so did he pray for Rivka daily as part of that? Or was this a one-time plea that God heard? If it was a one-time plea, did Yitzchak wait almost 20 years to make it or did he make it earlier but God delayed in responding? I am curious about when Yitzchak decided that prayer was called for and, after that, what he did and for how long before God responded.
Rashi describes his prayer but doesn't talk about the timing or frequency:
opposite his wife: This one (Isaac) was standing in this corner and praying, and that one (Rebecca) was standing in that corner and praying.
Finally, if this was part of his daily prayer, then does this Rashi indicate that Rivka, too, prayed daily?