It's largely open to character interpretation, within reason. Every character has the same level of "charge" in their dream battery, regardless of what their power levels would be in their own canons; thus, if the only thing a character has in her is a godlike attack, then she's not going to be able to use very many of them before she runs out her battery. The power of the attacks is variable based on the character's canon; their ability to fire them off is finite.

However, if you want a point of reference, then a good rule of thumb is that a character casting demigod- or god-level attacks is probably only going to be able to use one or two of them before knocking out, assuming they're attacking at full godlike power.