I have requested source code for a product that contains software licensed under GPL 2. The product includes a written offer to provide source code if you send a written request by email. The offer is valid for three years. But I have gotten no response. How much time do they have to respond until it is copyright violation? I didn't find that info in the GPL license.
The GPL does not explicitly specify a time within which the source code must be provided, which probably means a "reasonable time" is allowed. What is "reasonable" would eventually be evaluated by a court, if the matter ever got that far.
But please note that only the copyright holder (or the holder's authorized agent) can sue for infringement. The license does not give other people a right to sue for infringement, and I doubt that any license could grant such a right. One could inform the copyright holder who could sue, but the holder need not sue, and undertaking such a suit would involve expense, time, and effort.
Whether the offer to provide the source constitutes a binding agreement is not clear, and may well vary in different jurisdictions. The question does not state any particular jurisdiction.