Definitely a shorter time frame changes things. Trying to lengthen the time your customers stay with you sounds pretty irrelevant in your case (without knowing the ins and outs of your business).

Do you sell just the one course?

Some of these tactics could help you up-sell them on other offerings of yours.

Alternately focusing on getting more customers who abandon your funnel could be a quick win. See last week's post: http://blog.getvero.com/how-flightfox-doubled-their-remarket...

chris AT getvero.com if you want to go into real depth / just have a chat: I'd love to help and get you some more sales personally.