'By Milestone' is ideal if the duration is long (more than 2 weeks). In this case, Artist can specify how he/she would like to complete the Project in staged manner. There are no rules as such in defining the milestones. Artist has got full freedom to specify any no. of milestones, duration between each milestone, amount for each milestone etc. But, Producer may negotiate on the same terms with Artist before finalising the Project to Artist.