How can I control when people can access my survey?

You can publish your questionnaire by simply clicking Publish on the upper right corner of the questionnaire editor. Once published, anyone with the URL of the questionnaire can view it and leave responses, unless you set a publication start date. You can do this under the Publication tab in the questionnaire settings. Here you can also set a publication end date, after which the questionnaire no longer accepts responses. Please note that the start and end dates in an unpublished (draft) questionnaire will not be activated until the questionnaire is published.