I send rpc calls to a node through
tezos-client and I would like to know the length, or the height, of the blockchain so I can see if it is synced and, if not, how much time is remaining. How do I do that?
You can also call
tezos-client get timestamp repeatedly: it will give you the timestamp of the most recently received block. You'll get an idea on where you're at and how long you have left to synchronize. If you want to synchronize the full state of the blockchain, this might take a long time. Look into snapshots to get into sync faster. You can find snapshots here.