I have been asked to compile a report into dropout rates during checkout for a global webstore

I have used a sample size over one month as my sample because:

1. google analytics slows to a crawl over larger sample sizes and makes much of the analysis agonisingly small
2. I believe it to be statistically significant and a representative sample

My client has asked me why I didn't use yearly figures and wants proof that one month of data is 'statistically significant'.

Am I right in thinking that I need to compare the standard deviation of my monthly sample to the yearly sample and ensure that the deviation is under a certain %age?

Question: how do I prove one month of Google Analytics data is representative to one year worth of data?

Stats:

• 90k unique views/month
• ~1.1m per year.