This really depends, on the specifications and who is working to these specifications. If, for example, the specifications are relatively simple and are being performed by a professional luthier who knows what they are doing, then this procedure should not take very long at all, though I have no doubt they will want to take their time in such a delicate procedure. If, inversely, the specifications are very specific and the luthier is in fact merely a hobbyist, then the procedure ought to take a considerable amount of time, allowing for deliberations and doubts to mar the progression of the process.