The 5Nd is a number of shares, not a length of time.

You can convert it to a time if the pool hash rate never changed, but of course the pool hash rate changes all the time.
That time is how long 5Nd would take at that pool hash rate and if the hash rate didn't change.

A pool hash rate is (usually) measured in PH/s so lets say the pool is P PH/s
So if 5Nd is X (as shown on the web site it is currently 17,555,302,764,498.5)
Then at a fixed P PH/s a 5Nd of X will take:
(X * 2^32) / (P * 10^15)
or in that case (17,555,302,764,498.5 * 2^32) / (149.49 * 10^15) = 504,378 seconds = 5.83 days

I have a post about how you can estimate how much of the 5Nd you've been submitting shares ...
https://bitcointalk.org/index.php?topic=789369.msg29218581#msg29218581