I had a client ask the other day about how long you can make a survey.
My philosophy is always to keep surveys as short as possible to be respectful of the customer's time. It's hard to come up with a hard-and-fast rule for how long is too long, since there are many factors which go into a customer's willingness to take a survey.
The refusal rate for any given survey can be affected by the length of the survey, the perceived intrusiveness, the customer's relationship with the company, whether the customer had a very positive or negative experience, whether the customer thinks the data will be taken seriously, any compensation offered for taking the survey, and many other factors.
What's more important than the actual length of a survey is how long the customer thinks it will be. In our experience, once the customer agrees to take the survey she will rarely give up before the end (at least with a live interviewer) unless the survey is much longer than promised. People don't hang up at five minutes 30 seconds on a five minute survey (though don't push your luck too hard--promising five minutes and then doing a half-hour interview will probably bring bad results).
All these factors make it hard to tell exactly what the relationship is between the length of a survey and how many people will take it, but we do have a little bit of data. A while back, we were going through a survey design process for a live interview call back. The first version of the survey was quite lengthy, and the interviewers told customers that it would be a ten-minute survey. With that version of the survey we had a refusal rate of about 25% (the refusal rate is the percentage of people we attempted to survey who, when contacted, refused to take the survey. This is distinct from wrong numbers and people who never answered the phone).
After running that survey for a while, we convinced the client to take a fat red marker to the script and cut out a lot of redundant questions. The second version of the survey was about half the length of the first version, and interviewers told customers it would be a five-minute survey. With no other changes in the survey process, the refusal rate dropped to about 15%.
So going from ten minutes to five minutes in this particular survey cut the refusal rate from 25% to 15%. Other surveys will be different, of course, because of all the other factors involved, but this tells us that there is a significant but not overwhelming percentage of customers who will take a five minute survey but not a ten minute one.
Intuitively, I doubt you can get much lower than a 10% refusal rate without taking extreme measures (which might backfire), so in this case, making the survey shorter than five minutes probably would not have boosted the response much.
As a rule of thumb, for live phone interviews, I think that any survey that's five minutes or less is a good length and won't benefit much from being made shorter. Surveys much longer than five minutes will start to see a drop in response rates, so how long is too long depends on how much survey response you're willing to trade off for whatever additional data you're gathering.