Turn around times vary, dependent on the following:
With your quote, I will give an estimate in the time it will take me to do your work. Once I have reached the amount of work in the project that I have estimated - I will notify you and let you know if I am able to continue with the quote as given or if I will need additional time to finish the job.
This sometimes occurs when doing custom work and the amount of the work changes because the client's vision or request changes. For example, I am asked to create a logo with a bicycle and I comply, then the client changes their direction and wants the bicycle to be a motorcycle. In essence the work has begun and time has been invested in one particular area. This has therefore established a work product, but to redo or rework will require additional work not estimated at the onset of the job.