Final answer:
The Great Rift Valley filling with water depends on significant geologic shifts, such as sea level changes, not detailed in the given information; thus, we cannot provide a timeframe for this event.
Step-by-step explanation:
The occurrence of the Great Rift Valley becoming filled with water would depend on various geological processes. As per the details provided, the Great Rift Valley will not fill with water until and unless there is a significant geologic shift that allows for the flooding of the region either by a rise in sea level or other substantial changes in the Earth's topography which would allow ocean waters to enter the rift. Notably, there is no specific timeline stated in the passage for the flooding of the valley by sea level changes. A timescale of 30 million years is mentioned, but without context, it is unclear if it relates directly to the filling of the Rift Valley with water.
Relying solely on the available information, it seems that the final answer would be subject to further geologic activity that is not detailed within the text provided. Since the information needed to make a definitive conclusion on when the Great Rift Valley will fill with water is not present, the final final answer in two line explanation should be considered speculative at best and would be something along these lines:
The Great Rift Valley could potentially fill with water if there are significant geologic changes that lead to an incursion of sea water, but based on the given data, it is not possible to provide a definite timeframe for when this might occur.