A Hackathon is an informal gathering of people with a related interest in the goal of creating usable software. A Hackathon will usually have a theme, of which some examples include:

• A specific item of technology such as an API or a programming language or framework.
• A specific subject matter, for example, open government.
• Sometimes it's about the demographics of the programmers. Personally, I like the grey-haired C programmers' Hackathons the best.
• Or it may just be about a group of developers getting together to share ideas and show each other some cool stuff.

As well as a theme, Hackathons will usually have categories and there are usually prizes for the entry judged best in each category. For example, here are a few of the categories for a recent open government Hackathon in New Zealand called GovHack NZ 2017:

• Re:Invention (International)
• Tourism Hack
• Geospatial (Location) Hack
• Most innovative hack using Stats NZ data
• Data Journalism
• Making Donations Easy
• Search and Rescue Comms
• Sustainable New Zealand

So yes, you will need a judging panel. If you're doing an internal Hackathon, you can ask for ideas for categories from around your organization. You can then ask those who suggest a category that is used if they could be on the judging panel for that category.

The number of days dedicated to a Hackathon really depends on what you're trying to achieve. For public Hackathons, they usually require a single day or an entire weekend. It's an opportunity for teams to learn how to work together.