Can I stay on Drupal 7 forever?
IF the current version of Drupal is working out for you and you don't need more features added than you can stick around on 7 for a long time.

The problem is that D7 will not receive any feature updates and most modules will not be updated for D7 aswell. 

It is hard to say the exact date of when D7 will not be supported anymore but most likely in 3-4 years.

but the bug/security fixes will continue till the end of this year most likely.

source: https://www.ostraining.com/blog/drupal/drupal-7-end-of-life/