When generating project plans or timelines, never consider a resource as being 100% available. Most tracking tools allow you to put in the resource percent allocation. Even if someone is dedicated to working on a project full time, this does not mean they are 100%. Meetings, code reviews, and general interruptions take up time. Very rarely do I consider a resource as being available more than 80-85%. Often, seasoned developers are those who get interrupted the most, so their percentage needs to be lowered accordingly.