A few hours are sometimes enough to start generators when renewable energy supply decreases. Obviously, the more capacity the better, but costs will increase linearly with capacity in most cases.
Pumped-storage hydroelectricity - where it is feasible - is the only kind of energy storage close to "months".
You can store energy for months pretty easily as chemical energy. Just get some hydrogen, then join it to something else, maybe carbon, in the right proportion so it's a liquid at room temperature making it nice and easy to both store and transport.
Wait a minute...
Oh: pumped hydro is not a "months" storage technology. The capex per unit of storage capacity is far too high.
The point is that's already a well-served market. These competitors are like alternative semiconductors going up against silicon.