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 ?