The OP is likely getting the “15 minutes” from the general Catholic assumption that Jesus is physically present within us for 15 minutes after the Eucharist is consumed. It may be that some catechist taught that because Jesus is with us for 15 minutes then we should be praying and giving thanks for 15 minutes. However, this is not an official teaching of the Church. The result is that I have seen Catholics who receive and then leave right after Communion (and some churches are forcing people to do this as a COVID procedure), others who receive, stay for the final blessing, and then leave as quickly as possible, and still others who will stay after Mass for some length of time ranging from a couple minutes to 15 minutes to thank God, pray in their pew or in front of the statues/ shrines in the church, light candles, etc.
It is okay to pray for 15 minutes after Mass but it is not required, and in some churches (particularly in COVID times) they might not even want you in there after Mass because they need to sanitize the pews or they don’t want people gathering indoors.
I have also seen the case in some churches where a priest will come into the worship space to talk to parishioners after Mass. Other times they want you to go out to the vestibule or down to the church hall to talk. It’s kind of a “when in Rome, do what the Romans do” situation. But if you are talking in church then at least move away from anyone who is trying to pray. I don’t think people realize how distracting it can be to be carrying on some loud conversation right at the elbow of someone who is in prayer.
As someone else said, churches differ in what they encourage in their worship space after the Mass.