It means that the two (or more) people will have 9 hours to meet over the period (length of time) of 3 days. That is, 9 hours within 3 days. Or, out of 3 days time, they will have 9 hours of time together. It also implies that the 9 hours is spread out over the 3 days (for example, 3 hours each day). Or, mathematically speaking, they will be together for 9 out of 72 (3x24) hours.