Two things limit the real number of spells a character can cast.
#1: "The maximum number of Spells a character may cast during
his turn is equal to the number of Spells he had at the start of
that turn. A character may only cast one Spell during another
character’s turn." (p. 13)
#2: Any spell cast must have a legal target. So you can't cast Healing on yourself if you're at full Lives just to cycle a spell. Spellcaster with 1 spell which is the Counterspell, in a game where the other chars are non-casters, might be stuck with the spell for a very long time and unable to spell-cycle.
That said, you can still get very good mileage out of the spell deck, someone like the Warlock, especially after he gets +1 Craft, keeps drawing to 3 spells start of his turns and can potentially go through much of the spell deck even if just cycling by himself.