The time required just dictates how large the underground storage caverns should be, not how much the turbines cost. There is enormous space for these, and they can be very cheap. As you may know, natural gas demand is seasonally leveled using such caverns.
Well, natural gas storage is months worth, so that capacity would be there if we needed it. It's going to depend on latitude.
We can look at a model to see how much long term storage is optimal (under various cost assumptions) to deliver a constant output, using historical weather data.
For the US, the minimum cost model using 2011 weather data and 2030 cost assumptions uses 6 hours of battery storage and 106 hours of hydrogen storage. For someplace at high latitude, more hydrogen gets used, since it becomes economical to save energy from summer to winter. In Finland, for example, the optimal solution has 2 hours of batteries and 275 hours of hydrogen. On the other side, the solution for Saudi Arabia uses 10 hours of batteries and 48 hours of hydrogen.
