mumblechum isn't quite right - the Pre-Action Protocols indicate that a reasonable amount of time for a potential defendant to respond to a letter before claim is 14 days. OP, here is what you need to include in your pre-action letter (taken from the pre-action protocol which gives guidance on how to handle a potential claim):
"The claimant?s letter should give concise details about the matter. This should enable the defendant to understand and investigate the issues without needing to request further information. The letter should include ?
(1) the claimant?s full name and address;
(2) the basis on which the claim is made (i.e. why the claimant says the defendant is liable);
(3) a clear summary of the facts on which the claim is based;
(4) what the claimant wants from the defendant;
(5) if financial loss is claimed, an explanation of how the amount has been calculated; and
(6) details of any funding arrangement that has been entered into by the claimant.
The letter should also ?
(1) list the essential documents on which the claimant intends to rely;
(2) set out the form of ADR (if any) that the claimant considers the most suitable and invite the defendant to agree to this;
(3) state the date by which the claimant considers it reasonable for a full response to be provided by the defendant; and
(4) identify and ask for copies of any relevant documents not in the claimant's possession and which the claimant wishes to see."
You should ask for a full response within 14 days and indicate that if you do not hear from the defendant you will be issuing a claim against them. If you don't receive any response or acknowledgment of your letter within that time you are free to commence a claim and the court will consider you to have complied with the protocol.