Let's say I have Ethereum client connected to peer network none stop or mining, can someone identify how long I've connected, based on my wallet address some how? is it already built in? or we need to create a contract for that. Thanks.
Your wallet address will be randomly distributed, so just by looking on the address alone you can't see how long you have connected. By analyzing the blockchain you could see if and when this address was first used for mining (it will be the `coinbase`); but this currently isn't programatically exposed in any way.
If you rely on someone to 'check in' to the network, it is probably better to have them specifically register with a contract for this purpose (just like what will be done for the BitVote project).