How many troops form a "large enough" force?

[Amhazair]

I really had the impression the point at which gems start to be used was lowered several patches back, (I can't remember which one exactly) but since nothing of the kind was mentioned in the patch notes I suppose it could just my imagination.

Generally though, since that point I never had a situation where I wished my mages used gems but didn't. The other way around however... In my last game a large R'lyeh army (~750 chaff, some quality troops, 40+ mage communion, 1 Tartarian SC, and maybe a dozen other assorted mages) blew around 30 gems against 25 Pythium PD. Including a will of the fates on turn 2, and then, unscripted, a second will of the fates to give luck to the legionaires I had master enslaved after the first cast. That was funny... At least the second and third master enslave I had scripted were deemed unnecessary though.