With all my matings, be it Indigos, corn snakes, kings or pythons i put them together once per week for a few days, separate them to feed both if they will eat then put them back again for another 5-6 days.
I keep doing this until the female is obviously gravid. This method has worked many times over the last 20 years of breeding. I Dont always get to observe actual copulation as happened this year with my Giant Madagascar hognose but it works well for me.
I observed my Indigos this year mating several times & as Bob said one good session should do it but it does no harm to let them have a few sessions.

Good luck
Andy.