This depends on a number of factors. Amongst others;
1. The complexity of the question. It is more time consuming to figure out how to approach the problem of a declining profit company with increasing sales volume once you have received a data dump than suggesting a P&L structure if you are hardly given any information other than the fact that the profits are declining, for example.
2. What the interviewer is looking to get out of you. Maybe she wants to know how you react under pressure and therefore encourages you to present the first thing that comes to mind. Perhaps he wants to see if you can be trusted in front a client if you are put under pressure.
Nevertheless, often between 30 and 90 seconds is completely acceptable. But as I said, this depends significantly on the actual question.
Please don't hesitate to contact me if you have any further doubts.