Hi Linn, Katrina from WorldNomads.com here.
If you finish your trip early, we don't currently give a refund on the unused part of your insurance, unless you cancel it within the cooling off/free look period (which is explained the the full description of cover/policy wording).
If you think you might only travel for 2 months, then I suggest you buy a 2 month plan, and then you can extend your trip and buy more cover online through your World Nomads membership while travelling (you'll even get a 5% member discount if you do this).
If you have any other insurance related questions, please ask our customer services team direct - http://worldnomads.com/contactus.aspx as they can go into detail on coverage, which will depend on your country of residence.
Safe travels about 5 years ago