It's an interesting Question but needs one more bit of data to make it possible to get a realistic figure IMHO.

That would be how many of the rebills you are counting in your 'second' month were in actual fact 1st time recurrers.

I have many members with me that have been with us since they first joined one of our sites 2 yrs ago.

I know my overall figures of new sales to rebills works out at a totally different percentage to my monthly income from 1st Timers/ Rebills.

No that I'm re-reading my answer does it make sense? LOL

Basically I retain 38% of my customers long term (longer than initial sign-up plus 3 months rebills)
But My 1 time Rebill rate is up at 82%
& My 2 time Rebill Rate falls to 60%

I found it can be directly linked to updates, PLUS one really weird thing...You can update a site too often! 'Go Figure!!!!!' Its weird but true!