"What counts as a 'Spell On'?" has come up as a question a few times in my game.
One player's quickie definition was, "one you can maintain, but also Great Haste."
That's not a bad quickie definition for most purposes (especially the "don't stop and look up the rules now and try to learn them, we're playing here!" purpose.)
According to GURPS Magic, "temporary" spells count as spells on.
According to Spells, spells that specify a finite duration count as spells on. There are exceptions, and it lists them on pages 11-12.
Pretty much, if the spell has a non-permanent effect, non-instantaneous effect, and doesn't specifically say it doesn't count as a spell on . . . it counts as a spell on.
Great Haste? Sure, 10 second duration. Spell On. Flame Jet? 1 second duration, spell on. Fireball? No - you can't cast while holding it, and it launches and flies to where the dice say you threw it and does damage and it's over - so not a spell on.
A few spells we use do specifically need some call-outs:
Create Servant - Spell On. Finite duration - just because the wizard with this spell can maintain it for free doesn't mean it's not a spell distracting him when he's casting more.
Continual Light - Not a Spell On. It has a duration, but specifically gets called out as not counting.
Great Haste - Spell On. As mentioned before, it lasts 10 seconds and cannot be maintained but only the first part - a finite duration - matters.
Summon Elemental - Not a Spell On. The effect is over when the elemental arrives. Control Elemental is a spell on.
I'm sure I'm missing some common casts, but generally the players who use those spells know the rules. It seemed worth bringing this up here so I can list the spells that might get exceptions (or specifically need noting that they do not.)