This is not accurate. No one reached C2 in Spanish in 1500 hours. I also do not match anywhere near the hour resource recommendations either. This does not make clear when to read and how much but assumes reading is growing alongside of listening. This output is not really valid. I would recommend checking out Lejkos’ updates (last one found here) to get an idea of what a dreaming spanish approach can get you.
From what I have seen, doubling the hours seems like an accurate general rule, but it is not as simple as 300 spanish hours = 600 japanese hours. The earlier grind seems more like 3:1 instead of 2:1, but near the end of the 3k hours, it starts to look more like DS Level 7. Basically, the pain is frontloaded.