English native here. The former is correct: It's ready in under 30 minutes. The other way sounds weird. I'd understand it, but it doesn't sound right. I can't explain the exact grammatical reason, but things are often done (though not always) IN an amount of time, so I assume it's following that convention.
"I'll be there IN 2 hours"
"Meet me here IN ten years and we'll compare notes" (You could also say "meet me here a decade from now" where there's an implied but unspoken IN - "meet me here IN a decade from now").
"IN a thousand years, we'll all be dust."