Interesting challenge. I would first suggest that it is a math problem before an Excel problem.

Easiest approach might be to correlate the entire data set to an exponential curve (y=AB^x). This can be done using the LOGEST() function in Excel (http://office.microsoft.com/en-us/ex...in=HA010277524). Once you have the coefficients A and B, then you can use algebra to solve for x when y=1000.

Another approach might be to extract the "maxima" from the data set, and perform a similar analysis. The real difficulty here is how to identify and extract the "maxima" into their own table so you can use the LOGEST() function to regress that curve.