If I watched every (or very many) episode of the sitcom 'Friends' in one weekend, for example, I would say I had had a Friends-a-thon. This is a guess as to the meaning because we really need to know the context. We add the suffix '-athon' to a word to suggest that we did something for a very long time, or something lasted for a long time, without a break. It comes from the word marATHON.

Claires answer is correct. A phone-a-thon would more likely be called a telethon - an event that goes on for at least one day in which volunteers sit by phones and take calls from people making donations to some worthy charity. A walkathon is an event, also for charity, in which people will collect charity pledges from givers which depend on the amount that the participant takes part. On the day of the event, the participant (along with many others) will walk as far as they can, then get a slip signed by a judge to confirm how far they walked. After the event, the participant will go to the people who signed the form and receive money based on the distance they walked. You are probably asking, why not just donate the money. A walkathon generates more publicity and more people are willing to give money for charity to a friend or fellow office worker. I get asked twice a year by co-workers to pledge money for a walkathon that they will take part in