Most Helpful Girl
As soon as possible :), that's how you 2 will get to know each other as well as bonding, getting comfortable around eachother. Don't wait.
Most Helpful Guy
As soon as possible. The longer you wait, the more likely she is to have forgotten you. Call or text the same day, couple hours later if possible, or if you know how to talk to girls, then right after your conversation stops (assuming it was less than 5 minutes, if it was a long conversation, don't text her fast, give her a few hours).