What's the speed that blocks regenerate from being hit?

What's the speed that blocks regenerate from being hit?

The Wiki on Breaking and Ticks isn't helping?

Per the wiki on breaking, there isn't a regeneration speed, only a digging speed:

The player's digging speed is controlled by three factors: the block being broken, the item the player is currently wielding, and the mining penalties affecting the player.

The regeneration you see is purely visual effect.

Overall, this is a simple but variable heavy calculation, and the wiki does a fantastic job on explaining each piece to the digging speed puzzle. For your convenience, I've brought over their pseudocode for calculating the digging speed:

if (isBestTool):
  speedMultiplier = toolMultiplier

  if (not canHarvest):
    speedMultiplier = 1

  else if (toolEfficiency):
    speedMultiplier += efficiencyLevel ^ 2 + 1


if (hasteEffect):
  speedMultiplier *= 1 + (0.2 * hasteLevel)

if (miningFatigue):
  switch (miningFatigueLevel):
    case 0:
      speedMultiplier *= 0.3
    case 1:
      speedMultiplier *= 0.09
    case 2:
      speedMultiplier *= 0.0027
    default:
      speedMultiplier *= 0.00081

if (inWater and not hasAquaAffinity):
  speedMultiplier /= 5

if (not onGround):
  speedMultiplier /= 5

damage = speedMultiplier / blockHardness

if (canHarvest):
  damage /= 30
else:
  damage /= 100

# Instant breaking
if (damage > 1):
  return 0

ticks = roundup(1 / damage)

seconds = ticks / 20

return seconds

All of the information you need to populate the variables is available through the aforementioned wiki.

From personal experience, if you don't break it on a single mining "session", the block returns as new. The regeneration is instant.

You can clearly see it on harder blocks like obsidian, if you stop just before it breaks and start over, you still need those 8-9 seconds to break it.