Many iPhone 6s and iPhone 6 plus, even on iPhone 7 users are facing issue in restore that restoring iPhone 6S from iCloud gets stuck in the very beginning saying "time remaining: estimating ...". Restore from iCloud is estimated at 2 Days or may be as low as 1min but still climbing. The blue bar is stuck at less than 15%. Even with high speed WIFI the restore is still stuck for many hours and never seems to end.Usually this should not take more than 2-3 hours for restore to happen completely but if it still happens:
How to fix restore not completing in iPhone 6s/6+, stuck at calculating time:
Fix 1: Restart the restore from iCloud
For iOS 10 onwards , Go to, Settings -> User's Apple ID -> iCloud -> Bakcup -> Stop Restoring iPhone. And now try again with restore. It may work this time.
Fix 2: Sign out and sign in again
Turn off the phone and turn it back on, then skip iCloud sign in by pressing "Sign in later", once your on the home screen you can go into settings and log in and it works fine.
Fix 3: wait for sometimes
When the message 'time remaining: estimating ...' is shown, just be patient. It took my phone something of a hour, but eventually it came through and is now restoring perfectly.