How often should this be done?

 

Got the garage to do this for me 6 months ago when i got the car but i'll be changing the two rear tyres soon so not sure if i should get it done again or just wait until the front's are due for a change?

 

Only done 3500 miles since i got the car.

http://www.michelin.co.uk/tyres/learn-share/care-guide/wheel-alignment

 

"When you buy a new set of tyres and want them to last as long as possible"

Unless they're a really expensive set of tyres the cost of the alignment would outweigh the savings in tyres so unless you notice a problem that could be caused by the alignment, never.