I disagree with Thomas - communication problems are able to pull the project down very drastically. No solution to this problem can lead to serious delays, caused not only by the time of waiting for a reply. The answer may not be accurate as well, or not everyone got it.
Very often in offers for the customer I include a chapter about communication channels. There are clearly described methods of communication, which will be used during the project, and under what circumstances you should use them.
There is no information about response time, but are described procedures about how to change the channel of communication, if given (eg e-mail) starts to be problematic.
Records about the time appear in the maintenance or development agreeament. If the information is incopllete and you sent request to the client, the response time (specified in the contract or SLA) is suspended until the client response to the questions provided.
Under this assumption, I have never had a problem with some delay, because date of delivery was dependent on the customer response time.
The contracts often contains information, that when there is no end information about the project (such as end of testing or product acceptance) by the customer, after a certain number of remindes, we can accept them for the customer. Often quoted mobilizes the client to act.
It's good to create a communication plan (most project management methodologies recommends it). If required you can put there an information about response times. All interested parties should sign below it.
If you have SLA, there you can determine response times for both sides, or a statement that the reaction time is dependent on customer response. If the SLA does not exist, such records should be included in a contract.
For new orders, or extensions, you can put the information that if there is no response in the context of reception of the work, you can accept work for a customer and demand payment.
All these things you should have on paper, signed by all parties. Then you have something to refer to.
If you don't have such rules written anywhere - you can only fight with the customer.