Have a read of your agreement. There should be a section called 'Termination; Your rights' where it will tell you whether you have got this type of agreement or not.
Basically, it will say that you can terminate the agreement and the Finance company is entitled to the return of the goods in a reasonable condition and half the total amount payable as stated on the agreement. It isn't so much a time thing, although if you are on a 4 year HP agreement, it will probably tie in with this.
It is part of your Consumer rights and doesn't affect your credit rating.
Hope this helps...
JJ