Well it's that width of the gap, but in Nd, not time.
So it simply depends on the pool hash rate how long it will be to complete the 'ramp' back up.
But really what's going on is it always takes 5Nd to change your hash rate (up or down)
The 'tank' concept is actually easiest to follow:
It's how full the 'green' tank is when we find a block (the Red line)
Each new shift, the line on the left drops out of the tank, and the new line on the right effectively replenishes it.
The difference between the two decides what the red line will do.
So at the moment your total 5Nd hash rate isn't changing much since on the very left it's similar to the right.
Once you hit the empty area on the left where nothing is leaking out, your hash rate on the right is always above the left (zero), so the 5Nd will start to go back up again.
Looks from that graph, less than a day at the current pool hash rate and your red line will be going up much faster.
Edit: the above is all to do just with the green area - on the left and right of it.
