i red that the block will reject the transaction at a point, is it true ?
Nodes are likely to forget your transaction. If yes how long it will take ?
The default time for a Bitcoin Core node to forget your transaction is two weeks as of very 0.14, I think. However, users can customise the time that their node takes to drop a transaction. In that case will it be sent back to the sending address ?
Once a sufficient amount of nodes have forgotten your transaction, you may be able to send the coins again as if you had never sent them in the first place. This is not being "sent back", but you will be able to use them again.