How much "Proof of work accepted by server" needed to generate 1 bitcoin (or a part of it) I am using Bitminter client, and I am confusing about how much I earn in one week while my system working overnight.

https://bitminter.com/ says "Mining one bitcoin with just a PC now takes millions of years".

What is the basic rule of calculation?

Thank you.
As a miner in a pool, you will be paid a fraction of the pools income based on how much you contribute to the pool, compared to the other miners. So it all depends on how powerful your mining setup is. The reason that using just a PC is so unprofitable now, is because dedicated hardware (ASICs) that is specifically designed for mining is being used, which is thousands of times more efficient. 
The total hash rate of the bitcoin network is nearing 7 exahashes per second according to blockchain. This means the difficulty of finding a valid block must be really high in order to keep the block time at an average of 10 minutes. The estimated time to find a valid block is proportional to the difficulty, something like difficulty * 2^32 / hashrate seconds. Now a home PC, with a hashrate in the tens or hundreds of megahashes per second (rough estimate, depends on your hardware) basically just can't compete, at literal billionths of the networks total power. You'll end up paying a lot more in electricity consumption than you will make from mining. 

There are a number of calculators available online, for example:


https://www.cryptocompare.com/mining/calculator/btc
https://www.coinwarz.com/calculators/bitcoin-mining-calculator