In order to set a reasonable timeline for project completion, all members of the group should meet up and discuss their time estimations for each step and why. If there is any trouble agreeing on a timeline, the team can reference previous projects that were similar to the current one. They can talk about the changes they could have made that would have let the prior project run more smoothly (ex: spend more time planning before execution). I believe that adjustments to the timeline can be made if the team overestimated the time required for the initial steps, as long as the whole team is on board after checking to make sure all requirements were met. This will create more time buffer for further steps in the design process.