after how many days start staking?
also need to have online the wallet 24/7 for staking or simple hold it at my wallet without having online
You need to have it online at least 15 days for coin to mature before it can start earning.
No it is not necessary to turn on your system 24/7 for stakingbut you have take backup of your BitConnect Coin in Wallet.
because if your wallet will loss then you will loss your bitconnect coin.if you have backup then you can restore your coin.
To take backup refer this page :
https://bitconnectcoin.co/guide/6/How-to-take-backup-and-restore-of-your-BitConnect-Coin-Wallet