When is the right time to let it go and move forward? Yes, we are talking about migrating to Drupal 8 version. Drupal 8 was released in November 2015, so it has been more than two years now. No matter what kind of website you have, whether you have an online shop, small brochure website or an extensive and complex website, if it's built on Drupal 6, it's almost urgent you move forward and upgrade it to Drupal 8. Why? The Drupal community no longer (officially) supports Drupal 6 since three months after Drupal 8 came out. That means that bugs are no longer getting fixed. Drupal 6 is simply long gone, finito, end of story.
So why go with Drupal 8?
Drupal 8 has so many benefits. But it is a little bit different to what you have been accustomed until now. For example, you need to learn PHP framework called Symfony. PHP engine is replaced with engine “Twig”. But Drupal has many improvements, like better integration with services, mobile design, better user experience for admins etc.
Is it upgrade or migrate?
If you are hoping that your new Drupal 8 site is going to be the same as your site on Drupal 6, that may not happen. With all the improved features, your site is going to get different, but better. For example, all contributed modules that you are using on Drupal 6 might not have an automatic upgrade path ready (yet). Hence you need to sift through the migration results. You might also need to settle that it will be simpler to manually copy-paste the content from your old website to your new Drupal 8 installation or going with a custom migration approach.
Drupal 6 sites can be migrated directly to Drupal 8 or if you think you will benefit from Drupal 7 vast choice of contributed modules. Drupal 7 will stay fully supported until Drupal 9 will be released. That gives you a chance to choose if you would rather migrate your Drupal 6 site to Drupal 7 or 8.
A new approach to upgrading
Upgrading your website from Drupal 6 to Drupal 8 uses very different approach than going from Drupal 6 to Drupal 7. When Drupal 6 site is upgraded to Drupal 7, the same database is upgraded from Drupal 6 structure to Drupal 7 structure. Now, if you want to upgrade a Drupal 6 site to Drupal 8, you first need to install a clean Drupal 8 site. You will then need to migrate your site configuration and site content to your new Drupal 8 site. This means, your existing Drupal 6 site remains wholly unaltered and intact.
Unfortunately, upgrading from Drupal 6 to Drupal 8 is not a matter of clicking a button. Depending on the complexity of the built, we can even say it's a migration rather than upgrade. Nevertheless, I encourage you to look ahead and embrace the novelties Drupal 8 brings and upgrade your website.