xpell: following a depressurisation a descent would be required to 14,000ft/10,000ft
depending on the oxygen fit. When I used to fly the North Pacific routes between Asia and North America the available alternates were often a considerable distance away from the planned route therefore additional fuel had to be added to the route calculation to take in to account the much increased fuel consumption at the lower flight levels required for the diversion. This would have been precalculated as a "depres. critical point" to the diversion. The extra fuel plus the unused onward fuel would have to be sufficient to arrive at the diversion with 30 minutes holding fuel remaining. In very broad brush terms the increased fuel burn at the lower levels compared to the cruise burn was in the region of +40%.
I expect other ppruners will come on here and quote much more rules and regs. ro you but that is the simple answer to your query. Hope that helps?