Drupal 6 will be maintained till the release of Drupal 8. Drupal 8 release is expected to happen towards the end of 2013. The current stable branch Drupal 7 will be maintained till the release of Drupal 9 (probably 2015). If you are running a production site on Drupal 6 then you will probably have to start looking at upgrading the site by the end of this year.
There are couple of different options open in front of you if you are looking to upgrade your Drupal 6 site soon.
If the site does not use a lot of custom functionalities and uses minimal contributed module functionalities and you are not really interested in contributing towards migrating Drupal 7 modules to Drupal 8 then you could just go ahead and upgrade your Drupal 6 site to Drupal 7. This could be done anytime you have the budget to do this. If you are interested in contributing towards migrating Drupal 7 contrib modules to Drupal 8 then you could wait a bit and then migrate your Drupal 6 site into Drupal 8 by the time Drupal 8 is released. You could start on the work a few months before the release date of Drupal 8.
If the site does have a lot of custom functionalities as opposed to contributed module functionalities then you should really be looking to migrate to Drupal 8. Work on this could be started as soon as we have an RC version of Drupal 8. You will however have to be prepared to contribute towards migrating some of the Drupal 7 modules to Drupal 8 as it would take a while before top 100 contributed modules have Drupal 8 versions.
If you are not sure about which version of Drupal you should upgrade your site to do get in touch with us and we can help you make that decision for you.
Do note that this is a time sensitive article written in May 2013 and the opinions cited in this article may not be relevant after a few months from writing this article. The concepts would still hold though. Contact us to get our recommendation on what version of Drupal should you be upgrading your site to.