1. It probably depends on your state and how competently run the exchange is. In my state, I applied sometime in early- to mid-February and was able to start on March 1, so it was pretty fast.
2. Yes, you will need to go through some process to continue your coverage in 2018. In my case, though, they already know that I'm on an ACA plan, so the renewal process is easier than signing up initially. I don't remember the details, but they basically ask if your income is still the same, and if you want to still continue with the ACA, and if you still want to stay on the same plan. If so, you basically just say "yes" and things continue on, perhaps with a somewhat higher premium. This enrollment period for 2018 will be from 11/1/17 to 12/15/17.