I do not think it is very difficult. The value of each share you submit is calculated with the following formula:
score
share = exp(t/C)
where t is round duration in seconds, and
C is a constant,
currently 200. This means the value of each share is exponentially growing, so old share have significantly lower value compared to recent shares. So if you stop mining while others don't, your part of total pool score for that round will decrease exponentially. You can read the original post about this reward calculation method here:
https://bitcointalk.org/index.php?topic=1976.msg50002#msg50002.
Additionally, based on measurement I have found that
each hour, t is reset to zero and your
score is divided by exp(3600/C). This is probably to prevent an arithmetic overflow (see
http://en.wikipedia.org/wiki/Arithmetic_overflow).
Hope this helps,
T
(emphasis by me)
(Also the whole pool's score is reduced by the same factor, not only yours.) :-)
[edit] (In fact, it's the pool's score that is the candidate for arithmetic overflow, not the your one...) [/edit]
Thank you very much for posting the constants that resulted from your research, Tibor!
I've noticed that the current round (19720) score had been "reset" at about 20 minutes round duration for the first time and then between 1h 21m and 1h 22m for the second time.
I've looked just at this round, haven't examined any other one.
[edit] Next round (19721) first reset at about 1 hour 14 minutes. [/edit]