how long does it take to download all the blocks for full node?
and how safe is qt wallet full node compared to the ledger?
As you have been told earlier it's depends on your internet connection, location and the type of device you're using. and how safe is qt wallet full node compared to the ledger?
For ledger it's absolutely not safe and you should always avoid them. If you need security, you have to go for an airgapped devices (a devices that has no internet connection). You can create one yourself by using an old device or new device, just disable the WiFi card and Bluetooth connection or possibly use a software like Tails USB to airgap your device. For the software airgap you have to turn off your wifi in the OS and for hardware you have to physically remove your WiFi card.
Assuming that you have a fiber connection or medium-speed internet, you need an average of 2 to 5 days to complete the synchronization.
If OP is in my location, it would probably take longer than 5 days, maybe up 10 days because from my location the internet connection is very slow the maximum is around 21.35 Mbps which is rarely seen except you purchase an IPNX connector that is very very expensive and the medium or average is around 9.55 - 10 Mbps. lol But why do you need to run a full node?
For me, I'm just tryna get more experienced. 
