You're an ambitous one, aren't you? Nothing wrong with that.
It depends on what size you want your mech to be, and what it's constructed from. Let's assume it's wood (the most common L'arile building material). Since the Cyarliel are trying to build more Tannanliels, let's also assume that the mech we're fabricating is a hefty Colossal IV in size, making it around 100 feet tall. I don't know about you, but I can't turn 100 feet into cubic footage in my head. But I can ask my wife.
My wife - who's taking college-level trigonometry for fun, I should note - went and found a plastic Cookie Monster. We dropped Cookie in a large measuring cup and saw how much water he displaced (about 150 mL). The we measured him (138 mm tall). And then came the annoying metric-to-imperial conversions. These took a lot longer than you'd think, and resulted in my wife asking "Why are you throwing ninjas at me?" at one point. But we persevered. In time we came up with 116,997 cubic feet.
That's a lot of cubic feet. You'd need 11,700 rounds to cast a spell to make a mech that size, which is nearly a full day of unbroken casting for one person. But it hopefully helps answer your question.
I should note that my wife, of her own volition, is off re-checking our math. If she comes up with a revision, I'll let you know.
DragonMech line developer, freelance writer, tall guy named Matt.