A standup meeting should be as short as possible. 15 minutes is the absolute max in my opinion, any longer than that it's just a plain regular meeting.

Some people divide the sprint team apart for standups. I wouldn't recommend it. It should be about 10 minutes for an average size team. One thing to watch out for is it's not a status meeting - just what is affecting the team and what's in your way to get the job done. Often times I see the product owner asking a lot of questions during the standup and that shouldn't be happening.

Yes, it is unhealthy to have discussions and debates during the stand up. Use the stand up to identify the need for discussions, and have the discussions after.

Not more than 15mn, which mean 1mn30 per person in a 10 person team to answer the three questions. If you need more time, you are either not focusing enough or your team is too big (in which case, you should split it into two teams). Daily standups are not the right place for debates, discussions or problems solving. Such things should occur outside the context of the meeting.

Ideally a standup meeting should be less than 15 minutes for a regular size team (7-9 people). If you find that your meeting is consistently running over - try to figure out why ... are there long debates or other discussions that could be sidelined. Make sure the meeting is focused on items relevant to the team and specific discussions are resolved outside of the meeting.

Some teams make it the responsibility of the Scrum Master to keep the meeting within the time limit - sometimes with fun penalties if they don't. ($5 in a charity or happy hour jar, etc)