Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Year | 1st Venue | 2nd Venue | 3rd Venue | 4th Venue | 5th Venue | 6th Venue |
| --- | --- | --- | --- | --- | --- | --- |
| 2007 | Sheffield | Mexico City | Nanjing | NA | NA | NA |
| 2008 | Tijuana | Sheffield | Nanjing | NA | NA | NA |
| 2009 | Doha | Changzhou | Sheffield | Tijuana | NA | NA |
| 2010 | Qingdao | Veracruz | Veracruz | NA | NA | NA |
| 2011 | Moscow | Beijing | Sheffield | Guanajuato | NA | NA |
| 2012 | Dubai | Beijing | Moscow | Tijuana | NA | NA |
| 2013 | Beijing | Dubai | Edinburgh | Moscow | Guadalajara | Guadalajara |
| 2014 | Beijing | Dubai | London | Moscow | Windsor | Monterrey |
Question:
how long, in years, has the this world series been occurring?
Answer:
7 years