each block is bigger than the last
This is not necessarily true. Some blocks are bigger, others are smaller.
so yeah, each block will take longer to validate.
Some take longer, some take shorter. But overall, there is a lot more transactions to validate in the most recent 3 months than there were in the first 3 months of bitcoin's existence
Once it's done it's done, though, unless you nuke the local copy.
Actually, every time you shut down, you'll need to re-synchronize to catch back up with any new blocks that have been created since you shut down. The longer you have it shut down, the longer the synchronization will take when you start it back up again.
Then you'll start all over again. It took me, give or take, about 48 hours to first sync the blockchain.
Your time may very depending on the speed of your internet connection, the speed of your computer, the amount of RAM you have, the speed of disk access.
I wonder this sync thing does ever end?
It should eventually catch up, but it will continue downloading new blocks forever as they are created, and you'll have to sync any blocks that you miss if you shut down.
It has used nearly 16 GB of internet traffic and the progress bar is less than 20%!!!
I'm surprised at the amount of internet usage. The blockchain when fully synchronized should be less than 15 GB on your hard drive right now.