I can say when it's not the right time, and that's in the middle of the current project! I've done it a few times, thinking I could juggle them all, and never did it until I left one alone and finished the other one in peace.
I agree that there should be a plan that ends with a goal, something like - "this site is going to take the majority of my time until it's able to do this and this, and then it'll be considered 'done' and moved down the production queue"