Once you have agreed objectives for an event, you can start to design an agenda. Remember that there are two main things that the audience is likely to be interested in:
The first is content, delivered by your speakers. People make decisions about whether to attend conferences based on the list of speakers, and particularly the keynote speaker. It is therefore essential that you start recruiting speakers well in advance of any publicity, to make sure that your programme is as full as possible. There is more about designing a programme here.
The second thing that people want is networking opportunities. These are delivered by your delegates themselves, in the time that you give them to interact. You therefore need to build in plenty of time for this.
After all, how many times have you heard someone say that the best part of any conference is the coffee breaks because of the opportunities to meet people?