I would say as safe as you can get in a online wallet, so you are actually under the chances of getting your funds hacked. Also, nonetheless less possible the private key of your wallet is owned by the company behind Coinbase, which means if they would want nothing stop them from transferring those coins to another account and then you could never recover them. So I would suggest transfer those LTC to your own compatible wallet.
Having coins in an exchange is only recommendable if you are an active trader, for HODLers it makes no sense IMHO.
Hope this can give you a hint of how it works :)
Thanks @santana33
So should I purchase some device to store those coins ?
I heard once that as long as you have more than 1 week salary in cryptos you should have them somewhere else, and I agree with that. I would say both a device and then print the private keys and store them in a safe, you never know it may be worthy hundred of thousands some day.