When Solana validator node becomes offline in mainnet, what are the current network penalties and after what period?
The validator misses out on block rewards and, along with their delegators, inflationary rewards for the duration of the downtime. If the downtime persists, they may lose delagators or be subjected to "social slashing" should it negatively affect cluster stability enough to concern the validator community.
if your validator doesn't cast a vote then it misses the reward cycle. There currently isn't a system for slashing but you can see at https://docs.solana.com/proposals/slashing the accepted solution. For now it's a "nothing at stake" system. you can see some of that discussion here: https://github.com/solana-labs/solana/pull/26294