Sometimes it’s helpful to estimate the length of a survey before it has been programmed. As a rule of thumb 1 minute per typed page for an online survey is a good start, for telephone multiply by 1.5 (and for translation to German multiply by 1.25).
If you want to get a little more technical… on average respondents can answer 5.5 questions per minute. So, you can simply tally up the questions and divide by 5.5.
But NO CHEATING! Remember to include all the screening and profiling questions, count each brand assessed against a battery of statements as a single question, add an extra “question” for every concept or lengthy instruction to be read and count every open ended question as two (they really do take twice as long).