How long does it take to break a stone block with an efficiency V (5) diamond pickaxe?

With a Haste II beacon, it would be instant mining, so you would break one block per game tick. That's the 0.05 seconds in @Penguin's answer. However, without Haste II, after you break the first stone block in 0.05 seconds, you will have a 0.3 seconds delay before the next block starts breaking.

Source: official wiki:

When breaking a block, a tool and its enchantments do its speed value as "damage" to a block every game tick, and when that value equals or exceeds the block's hardness, the block breaks. If the tool and enchantments immediately equal or exceeds the hardness times 30, the block breaks with no delay; otherwise a 6 tick (3?10 second) delay occurs before the next block begins to break.

For example, a player with Haste II holding an Efficiency V diamond pickaxe can break stone instantly, as the damage is (8+26)×(1+0.4)=47.6, which is greater than the base hardness of stone (1.5) times 30 (which is 45).

So, with an Efficiency V diamond pickaxe and nothing else, you would break the first stone block in 0.05 seconds, and the rest of them at a rate of one each 0.35 seconds.

Okay, so, according to my math, which is probably wrong, it is 0.05 seconds. Equation:

hardness = 1.5 (stone's hardness)
toolSpeed = 8 (diamond's toolspeed)
efficiencyLevel = 5 (level)
efficiencyBonus = efficiencyLevel ^ 2 + 1 = 26 (efficiency bonus speed)
----------------------------------------
Seconds: ceiling(1 / ((toolSpeed + efficiency bonus) / hardness)) / 20
-or-
Seconds: ceiling(1 / ((8 + (5 ^ 2 + 1)) / 1.5)) / 20
-or-
0.05

Equation info source: https://minecraft.gamepedia.com/Breaking#Speed