Sample size calculation issues


I have a question regarding a research project I'm working on. We have conducted an experiment where the same sample is measured over and over again to get an idea of the analytic variation in our system. We have a nice normally distributed data-set with all parameter estimates. We want to continue to use this sample in the future once in a while to check that the data is still distributed with the same mean and the same variance.
In order to do that, I want to use the least possible amount of repeat measurements in order to save materials. I've tried to calculate the sample size, but I'm pretty sure I'm doing it wrong. Can anyone help me with the tools necessary for calculating the minimal sample size?:confused: