how long does it take to download all the blocks for full node?
It depends on your system specs.
Having an SSD and adding more RAM can improve the speed of the syncing process.
and how safe is qt wallet full node compared to the ledger?
The safety of your wallet/node depends on you.
About the ledger, we do not know how safe this hardware wallet but since it's a closed source it's not safe. Having a Bitcoin core on Linux OS is much safer.