We recommend that you keep topics short. A single agenda item should answer a specific question. If you have very long topics, break them into smaller multi-part topics.
So, suppose your topic is: “Should our company use Instant Agenda for all meetings?” You can break this up into “Should we use a tool or product to make our meetings more effective?”, “What products and tools are available?”, “What are our top 5 requirements for such a tool?”, and so on.
If you aren't sure, start with 5 minutes for a simple question where most are likely to be in agreement. A more contentious or involved topic will require 10-15 minutes or more. Note: the moderator can always adjust this time, or add more time if needed.