Longer quests may take from a week to a month. Even if you are using a quest developed by someone else, you can tailor it to your needs or computer availability. There is no reason why you cannot design your own WebQuest. Keep in mind that the task must be achievable and that students may use non-Web resources as well. They will need some guidance on how to organize and how the project will be evaluated.
Some possibilities are a rubric or benchmarks to follow.
Be sure to allow time for reflection and application to real life. In my opinion, much of the fun in designing your own WebQuest is the product choice. Perhaps students will create a debate, diary, or scrapbook. Maybe they will write a news article, poem, or play. They might choose to develop a Hyperstudio Stack or electronic slideshow.