The Duration will depend on:
- the answer form (Yes/no, some number, a written answer, etc.)
- your participant
- how boring the questionnaire is
- how ambiguous the question is
Most of the time, "magic numbers" will be off by a fair margin
The best predictor of test duration will be a "pre-test". No first draft is perfect anyway. So you simply should present your first draft of a test to two colleagues and measure how long it will take them to complete it. Also you should ask them for their comments regarding some questions of your tests.
This will only take about 15 min + 2*Test Duration and is extremely valuable.
You will get an idea about the duration of your test and know how to improve it.