When providing services as a design professional, there are typically trade-offs required between your compensation, the amount of time allocated to you, and the level of detail that will be reflected in your instruments of service. Clients often want some indication about the amount of time it would take for your services to be completed so that they can plan their schedule accordingly. You may be able to do this with reasonable certainty, especially if the scope is well-defined and your assumptions about the project are correct. Your estimates, however, are just that—estimates. You should be wary of the client who asks you for estimates and then treats them as deadlines. It is up to you to clearly communicate to the client that your time estimates are intended to provide guidance based on certain assumptions; they are not intended to establish firm deadlines.
There are a lot of adverse consequences when working with short deadlines. You are likely to make mistakes. It leads to staff burnout, and there is a distinct lack of creativity when evaluating alternatives and providing solutions. Short deadlines damage accountability since the rush to complete provides a convenient excuse for mistakes. Once you lose accountability you lose the trust of the client, your most precious resource for your practice.