how long does it take to download all the blocks for full node?
The current blockchain size is around 550 MB and the time it takes to download that depend on your internet speed
and how safe is qt wallet full node compared to the ledger?
Your security depends on how you create your wallet and the security of your device.
Take note that any online device is always to prone hacking and with downloading the blockchain and running a full node, you don't increase your security.
People usually run their own node for helping the network decentralization and protecting their privacy, not increasing their security.