As for disputing charges on credit cards in the UK, there are two separate angles on this.

1) You "dispute" the charge. This is where you claim the charge is unrecognised or unauthorised. Here the only issue is whether you agreed the payment or not. Where there is no signature/PIN, credit card companies don't normally get into a discussion - they just refund back and grab the money from the merchant. (I've seen this from both ends.) Where there is a signature, its more complicated. At the moment, they are taking PINs being entered as conclusive evidence that you did authorise the transaction. (Not sure how long this is sustainable.)

2) You are not disputing the validity charge, but you are claiming you are owed something back - eg a refund in this case or perhaps "damages" in the case of a breach of contract - ie the supplier hasn't done what they said they will. In UK law, credit card companies are jointly liable with the supplier. If the supplier doesn't issue a refund for defective goods/services etc., you can hold the credit card company liable. This is a feature of the Consumer Credit Act 1974 and doesn't apply to charge cards/debit cards. Getting money off a credit card company via this route is more cumbersome - as they seek to avoid refunding unless they can be sure of charging back the merchant. Also there are some exceptions (eg purchases under GBP100, non-consumer purchases etc).

There are no strict time limits. In the case of (1), it is always for the credit card company to prove you owe the money, not for you to prove you don't. In the case of (2), you would have to act as soon as you reasonably could. If you've sat on something for a long while its unlikely a court would exercise discretion in your favour if it went that far.

No doubt the US is a little different.