"roughly" is dependent upon the changing pool hash rate.
You could probably replace "roughly", currently, with "not a chance in hell of being correct but at least it's better than no information"
The problem is knowing the "current" starting point for 5Nd, that I've mentioned before being not currently available, and an extra CPU hit on processing every share the pool receives, thus one of the reasons why I've not done it yet.
See the 2nd half of this post:
https://bitcointalksearch.org/topic/m.13438840
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The 5Nd is shrinking ... not suddenly shorter.
These lines on the Help->Payouts page are the relevant ones:
The ramp isn't missing rewards, it's delaying them to reduce variance.
Each share is rewarded in all the blocks found in the 5Nd after the share.
That's simply how it reduces variance. Each share's reward is averaged out over the 5Nd after it.
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Now back to your "curiosity"
So if you have 15TH and mine without turning off your miners you would expect your 15THs to reach "15THs in rewards" after a total of 5Nd shares have been sent to the pool.
Someone who joins the pool after you will have to wait from the point when they join.
How long is "your" 5Nd point after you start?
Well that's the mystery
But it will always be less as time goes on (until you reach it) and anyone who starts after you, will of course have more to go than you.
How much time will "your" 5Nd take?
Depends purely on the pool hash rate.