UPDATE: The reset time is 12am PST.
Based on comments and testing with multiple accounts it appear the drop rate for the Savanna Collection items is about 1 item per energy pool. The drop rate appears to be much higher on Ruby mastered jobs than it is on the Bronze level so staying on bronze for this event isn’t going to help reduce energy costs. It’s hard to determine for sure because energy costs for jobs in South Africa is based on your level so there could be some variations based on level and not just energy pools or mastery levels. There is also the unknown Zynga variable which if this event is anything like the Limited Time Mission there may be accounts our there that don't get any or have to spend way more than 1 energy pool. For the collection item in District 1 it took me about 7 clicks to get one item (107,800 energy) and my total energy pool is 100,828 and the job is ruby mastered. The second image is from an account that is mastered on the bronze level with a total energy pool of 36,506 and it took about 55 clicks to get the same item (29,810 energy). These are rough estimates and appear to match what many players are reporting. What we don’t know at this time is when the day resets. All we get from Zynga in the instructions is “Each item will drop for a maximum of once a day from the corresponding district.” but they don’t say when the day starts or stops. If we were to guess I would think it would be the Zynga reset time of 12am PST but it’s possible the items could start dropping again at the local reset time or the day could be on a 24 hour timer and reset from the time your item last dropped. We won’t know for sure until tomorrow. There isn’t an official FAQ so all the information we have to go on is from the instructions.