There's no golden rule but 21 days is often cited as a maximum reasonable period within which you should expect some type of positive response (By 'positive response', I mean something more than just an acknowledgement).
Hopefully, you will have kept a copy of your original letter. Make a further copy. Write another letter, something like this:
"Dear Sirs,
I refer to my prevoius letter to you, dated xx/xx/xx, of which I enclose a further copy. I have not yet received a reply to this letter. I therefore give you notice that, unless I receive a satisfactory reply within the next seven days, I intend to pursue this matter through the small claims court. Furthermore, unless I receive a satisfactory reply, I intend to express my dissatisfaction with the poor service of your company to the local press.
Yours faithfully, xxxxxxxxx "
Send this letter by recorded delivery. Then just wait for the grovelling phone call expressing sincere apologies and offering to put things right immediately. It always works for me!
Chris