I haven't seen any really thorough, so this'll be hand-wavey and I'm rusty on statistics.
People say it's ~2% chance to get a Giancryst, but then (I'm guessing) it's a 50% chance it's the one you want since there are two varieties in Wise. With a 1% chance to get one, you can expect to run it ~100 times* to get a Support one or ~1500 energy. That's what, five or so days of nothing but gem room runs?
(*not necessarily true, of course - but if you run it say, 50 times, you still have a 60% chance of no support giancryst - 0.99^50 Around 75 runs is where it tips 50%).
/edit: It looks like Reddit did have a
thread about this. Sounds like their conclusion was about the same.