....it would depend on the exact wording of the specific context.
"The full product must be returned to your dealer with proof of purchase within 30 days for a full refund."
http://www.dolphinuk.co.uk/dolphin.asp?id=54
"
If I change my mind...
Within 30 days
•Most things we sell are covered by our 30-day money-back guarantee, so just return them to us unused, in their original undamaged packaging, in a saleable condition, with your receipt and we’ll give you a refund."
http://www.argos.co.uk/static/StaticDisplay/includeName/Retu...
These are just two examples where "within 30 days" is stipulated, the insinuation being that it is quite possible that if you leave it later than 30 days you won't get your money back, no matter what.
However, the (30 day) guarantee of money back doesn't necessarily preclude getting your money back later than 30 days if they include additional clauses, there may be exceptions, with certain circumstances etc...
Also, after having worked in retail, I can tell you that some stores are more flexible on this than others.
The translation workplace 
peer agreement (net): +3
