How long does it take currently to sync the entire chain if you want to launch a full baking node (if you start from zero)?
To fully sync the entire chain from the start should take a little under 24 hours depending on the speed of your SSD. If you are using non-SSD it can take many days and is not recommended.
New history modes have recently been added as it's not necessary to regenerate all of the context metadata from the beginning for most cases. Running in those modes will sync from start much faster and be viable on lower cost hardware setups.
The new modes are summarized as follows for more details can read https://blog.nomadic-labs.com/introducing-snapshots-and-history-modes-for-the-tezos-node.html
History modes allow the node to run without maintaining the full archives of the chain.
Here are the three first modes:
full nodes store all chain data since the beginning of the chain, but drop the archived contexts below the current checkpoint. In other words, you can still query any block or operation at any point in the chain, but you cannot query the balances or staking rights too far in the past.
rolling nodes are currently the most lightweight, only keeping a minimal rolling fragment of the chain and deleting everything before this fragment (blocks, operations and archived contexts).
archive nodes store everything. This corresponds to the current behaviour of Tezos nodes.
Full nodes will be the new default, as they are sufficient for almost everyone. We plan to introduce new modes in the future.
An important thing to note is that running a full node is enough to maintain the full chain history. Indeed, archive nodes do not need to use archive peers to bootstrap their archive, but only full peers, as the chain data is enough to apply the chain and construct the context archives. In other words, the network does not lose any security by switching to full as the default.