ERP implementation process question

The CEO states that he is highly concerned about the timeline that the implementation partner has outlined in their proposal. He notes that 18 months seems like a long time to implement the ERP system, and is concerned about the impact on the firms seasonal business. In particular, he wonders if through a combination of additional business partner resources and temp hires the implementation could be completed in a more reasonable timeframe. He even suggests that a nine-month implementation timeframe would be highly desirable. He now shifts to you and asks you opinion.

Please consider the following:

1. How do you respond in general?

2. Are there any frameworks or rules of thumb that might support your argument?

3. If the CEO continues to press for a shorter timeframe, what do you do