1- How much time it will take if i restart solana validator service?
This could vary on a lot of things such as your hardware setup and snapshot intervals, but if you have a node that is capable of keeping up with the cluster, I have seen 5-10 minutes for v1.14.10 against mainnet (note that v1.14.10 is not yet the recommended version for mainnet-beta) to get back up to the latest state prior to stopping.
2- Can we save the state of solana rpc if it restarted due to any reasons so it can restart from the last position.
Yes, the node will automatically do this by creating snapshots. Snapshots are essentially save points that occur at regular intervals. Your node will continually create snapshots and replace the old ones with newer ones. When do finally need restart the service, it will reload the latest snapshot.