It doesn't stay charged. You push the more active reactants into it, it gets charged while it reacts the reactants into the less active version.
It is like a diesel powered generator, where you push diesel and oxygen into it, it creates power while it reacts them both. The difference is that flow batteries also do the reverse reaction, so you can get the original reactants back.
About how long it will stay charged, it does not discharge like a normal battery, but active reactants have a shelf-life.
Not quite. Lithium Ion is cheaper at home-energy scale.
Flow batteries are more expensive and complex. So they are ideal for utility-scale projects only. When you're powering an entire neighborhood or city with a single battery, you'd probably rather make a Flow battery rather than a million Lithium Ion batteries hooked up together.
It is like a diesel powered generator, where you push diesel and oxygen into it, it creates power while it reacts them both. The difference is that flow batteries also do the reverse reaction, so you can get the original reactants back.
About how long it will stay charged, it does not discharge like a normal battery, but active reactants have a shelf-life.