I think Know Thyself is the best situation here. I take lessons but do indeed plan to stop taking them within five years. But I also know I am constantly interested in new things so I may take up painting, or a new language.
Also, with those of us with small children at home have a lot more wildcards that may come their way as the years go on (especially if it's in your value system to pay for extras, like we do). I allocated a specific amount of every family member for the foreseeable future to cover these expenses. If anyone decides they never want to take another music lesson, great! More for saving. But I think it's important to find yourself flush over shortchanged so I think our goal will stay the same.
So given this, I make our current FIRE savings goal the bare budget of our current lifestyle and I adjust it up/down based on the current year. Because we plan to stagger our retirements, it will give us a good idea of how much we save/spend once my husband is done work. Unlike most people, he has zero costs associated with working - he works from home which means no eating out and no fancy clothes, and in fact his work pays his cell phone and our internet, so our expenses may even go up!