For the ministries that I assist through the application process, I first obtain their consent (via Form 2848) to represent them and be the key contact person with the IRS. After working with the ministry through state formation process, I complete the federal application for tax exemption. The time to assemble the complete application can vary, mostly depending on how long the ministry takes to establish its board, approve bylaws and other resolutions, and get a working ministry plan together.
Once the Form 1023 (application for 501(c)(3) status) is completed and submitted to the IRS, the IRS quotes 90 days for the EZ form and up to 180 days for the full Form 1023 as the expected wait time before being contacted. The total time required to receive the final determination depends on whether there are any complications or the IRS needs more information. The IRS has historically sorted the applications into three piles: one pile contains applications that are complete and raise no concerns, another pile has applications that are complete but raise some questions for clarification, while the last pile of applications are incomplete or have raised serious concerns about the nature of the charity. Whatever pile your application gets placed in will determine the timeframe for the IRS response.
Once you obtain a determination letter from the IRS recognizing tax exempt status, your tax exempt status is retroactive to the date of formation as a public charity (aka Nonprofit Corporation). In the meantime, you are able to inform donors that you are in the process of applying with the IRS for tax exemption, and the donors may take the usual tax deductions based upon that claim. (However, adjustments would have to be made on those donors' future tax returns should your application be ultimately refused.)