Huh... I guess you're right. Still, 14F gal
is very worth using in the main game, since Iku is the only character who can buff ATK/MAG and she isn't used by many people anyway. Against a boss significantly weak against debuffs (16F hinthint), having her use SPD down is almost more important then having her deal damage, although she can do both of course.
I'd rather just use
for SPD down then actually keep using Cirno, though. Unless it's for added challenge. After 12F boss, she's just not really useful enough; PARing mooks is nice, but Reimu can do that too (albeit slower), and will begin having the SP to do so often. I see why one might pull her out purely for the 16F boss, though. IMO, drop her after 12F triple boss, use Reimu for mook PAR (her regular damage will be getting kind of ehh anyway), pick up 14F gal for SPD-down and her other usefulness.
People that really love Cirno could probably continue to get mileage out of her for all of main-game, except for a much-needed replacement during the 18F/Final boss. I have to admit, her high speed growth and lvl-up rating helps make her the best person to be placing PAR or SPD-down on a group of enemies, even if she's made of glass and good for nothing else.
How long does a single level on the extra disk take?
If you mean floor, well, it depends on the floor! If you literally mean character level, you've gotta grind up to like 200 (you'll have a bunch of V2 bosses+Bloody Seals+Boss Rush to make this less boring) to get ready for 21F, then the natural encounters you face exploring each floor should accommodate the 10~ level ups you should get each floor to keep up with the enemies. Hitting 30F around lv300, where the game basically turns into "Grind until you can face the next 30F boss", of which there are three.
If you don't mind a more-then-decent bit of grinding, facing the first 30F boss (who is very epic-looking and sounds like a pretty cool fight, if I remember the bosses right), is sane. But actually beating the Plus-Disk Final requires doubling the level you were upon reaching the floor (aka hit lv600) which is... well, jeez.