How long for dinner?

I would give dinner at least 45 minutes to an hour, especially if you're doing a buffet.

You can do speeches during dinner.

Depends on how many guests you have and what type of food. Buffet style for 100 people? 30 minutes probably won't work. But if it's being served at the table or you have a smaller group you might be able to work it out. It still seems pretty short though. That all being said, speeches during dinner might not be a bad idea... everyone will be paying attention and seated at least.

We started speeches 30 mins into dinner to minimize boredom. Everyone had their food and the VIPs were finished eating at that point. It worked out great.