Right now, usually 2-6hours of nameplate capacity. This is all that's really needed to meet the evening and morning parts of the duck curve, particularly since most grids have hydro and wind as well.
However, if there's need for more, then the storage side could be expanded. Personally, I'd like to see a ton more expansion of solar and storage behind the meter, at people's homes and at industrial and commercial sites, so that we can reduce the need for transmission and distribution. The US stats for electricity costs are something like $0.13/kWh on average, with $0.05 of that from generation costs and $0.08 from T&D. Even if installation costs are slightly higher at smaller, more distributed sites, T&D isn't cheap either. And having more distributed generation and storage as the potential to greatly increase reliability, particularly after natural disasters.
It does sometimes get cloudy for days on end which reduces solar output to about 10-20%.