If you are, chime in!
The group class people have a 9-12 mo life span, longer if they get involved on the bike. My place is like Hotel California, once they check in, they never leave!Hi Integrate, I work for a digital marketing firm here in Minneapolis. I've run a number of referral based promotions for clients, and they all seem to work a bit differently, with varying degrees of success.
To answer a few of your questions, I think to determine the length of your promotion you have to consider the lead time between a referral and a dedicated client. Do people regularly come for one session and not come back?
I'd start by simply saying that each new referral has to complete a number of sessions. This will increase the stakes, and help close potential leads. By the description of your sessions, it sounds like a month or two might be enough time for someone to bring friends in for a few sessions so they stick around.
Also, I suggest making the playing field fair for all customers by stating each referral counts as one entry into a drawing. Often attrition is a factor in these contests because one person gains a large number of leads, and all others consider their chances of winning a prize diminished and eventually stop trying.
Further, think about a platform where you can track and promote those who are bringing in referrals. If you have a Facebook page, email newsletter, or website, reward and congratulate both the referrals and the regular customers for helping you out, and ultimately helping others with fitness and regular exercise.
Lastly, you may want to consider bundling a promotion for referrals into this contest. Give them discounted rates, or some sort of perk. This not only helps attract new business long term, but it also helps those who are bringing in referral to close the deal. Lessen the barrier to entry and more people will take advantage. We call this eliminating FUDD's - Fears, Undertaintly, Doubts, and Dealbreakers.
Hope that helps! Let me know if you have any questions!