I have always wondered how long Local SEO customers stay with an agency if they are doing ongoing work for them.
Great question.
The easy (and most accurate) answer is, "However long you continue to do good work for them." There's really no reason for anyone to leave if leads are coming in consistently at a price that works for them.
I've only lost customers when a) they were phased out of my pricing as I grew or b) the few times the SEO wasn't working for whatever reason (which sometimes happens). I've had clients with me since the beginning. I've also had a few drop off too quickly before the SEO started to kick in. I've also had a few leave because they were in hyper competitive markets and I couldn't get them where they needed to be after the 1st year and they wanted to try something else (almost all of them ended up ranking on the 1st page a few months after leaving, typically at the top, sucks :/ ).
Lifetime value for me, at this point, is minimum $100,000. I'm more confident in my abilities than ever before but when I was less confident, it could have been around the $20,000 mark. It just depends on your agency (margin) and how long you've been able to keep clients in the past plus your improvement rate.
As I alluded to earlier, I've got clients that have been with me since the beginning (close to 6-7 years now) and some that I just brought on, but expect them to stay the same amount of time.
Hope that helps.
And, which type of client have you found to be the stickiest for your agency? Referral (from existing client), New lead (found you online/event), Disgruntled (not happy with existing agency), Cold (prospected via sales/advertisement)?
I agree with Phil that someone who is familiar with you, has read your stuff, and is a fan is going to stick longer. That's 100% true.
I also think that they will stick around, no matter what, if you're doing good work. Are they seeing business from SEO? If so, they will stick. Really, client retention in SEO (and honestly, in most things) comes down to your product. Does your product work?