I have two potential projects and one project where the client is taking a long time to get back to me. I want to let one of my potential clients know when I can start work on his project, but if he takes too long and I start on one of my other projects then the date when I start on his will have to be pushed back.

How do you schedule multiple projects at the same time? How do you let customers know that their project could be pushed back if they don't act fast?