Promotion for extra membership months - tips and affect on retention rate?

Hi,

I'm thinking of using a promotion giving extra membership months. I'm wondering how many months extra you have given and have you tested the success of offers for say one extra month versus two or three? How long was the offer in effect? This could be a flash sale for say a week or it could be available for a longer time, like a month. Since I'm hoping to attract new members, thus promoting via eblast rather than on-site, I'm leaning toward a shorter time period.

Also wondering if the offer might affect affect retention rate. Since the report looks at the number of people with a given expiration date that renew there should not be a numerical affect, (as in say 100% were still active at would have been the 12 month expiration.) Of course there will be an affect on cash flow and may be the usual problem that members who join with a promotion may expect a promotion to renew.

Appreciate any insights you have!