I’m not sure I understand your question correctly. I think you’re asking how long it would take to earn back the Gem investment required to purchase the 60 Gem Habitats. But would that include the time it takes to breed the Gem Dragons? I don’t suppose you’ve already stored some 140 Gem Dragons in your Habitat…
But the straightforward, basic question: how long does it take to earn back a Gem Habitat is pretty simple: 75 weeks. Multiplying the number of Gem Habitats by 60 doesn’t change this: it will still take ‘only‘ 75 weeks to earn back this investment.
For the record: this does ignore Kairos.
But if you intended your question differently, please elaborate a bit. I do like puzzles like this even if I don’t think I will want to be spending the next three years breeding nothing but Gem Dragons (assuming it takes about a week, on average, to breed a single one).