Thank you for your answer.So this means that if you mining with power of 100 Ghs (and difficulty will not change) then, system will give "time to find block" always fixed amount for example 2 hours and this amount will not change even after 2 hours of your mining but meaning that "with this power usually will takes to you to find block after 2 hours".I mean after you start mining, your past mining work is not calculated and not stored in any memory to continue on it later.It is just an indicative number ?
Yes, it is just an indicative number.
To find a block there are a few parameters depending on it:
1. Block time (BT)
2. Total Hashrate (TH)
3. Your Hashrate (let's say this 'x')
4. Luck
Now, the time to find a block with x hashrate is : (x/TH) * BT
But that time is just an estimation.
For explanation sake, let's say with your hashrate it takes 4 hrs to mine a block but it could take more or it could take less than that.
Now here comes the factor of luck, if you are lucky you may hit blocks successively under the estimation time and your luck will be above 100 for that time period.
If it takes more time to hit blocks then it will be less than 100 for that time period.
But on a larger time scale like a month, the luck parameter will average out to be around 100%
Now about your question for work saved. The work is not at all saved and it will start calculating hashes again and it may take the estimated time to hit the block.
I definitely recommend watching this video for a better understanding:
https://www.youtube.com/watch?v=bBC-nXj3Ng4