Any plane built in a 12-18 month period is a amazing feat in my book especially if your working alone or with little help and sharing a shop with another business. I think there is two types of commitment's a person is up against. 1- is the plane type? for instance a Piper Pawnee needing a wing redone or a engine rebuild. If the plane is bread and butter for the family I have witnessed around the clock scheduling for repairs is needed to keep the customers from moving on to another plane and pilot. 2- would be a person's HBA that is being built for pleasure. If the builder finds scheduling is the best way for them to reach their goal then by all means that's the way to go. I myself take any free moment and build or study and always keep my eye's open for needed material's and have found that part's come at some of the oddest time's. I think any builder will hit a spot where the build will come to a stand still out of their control and to me that's the challenge of the builder to not let that get them down. If your patient and give the matter time to pass in time the craft will be done.