As some of the others said it really depends on various factors, so I haven't added a vote..
Most of our younger snakes - boids that are still growing - and all of the Colubrids are fed weekly, generally Sunday is our feeding day.
The older snakes - particularly Boids 3-5+ years old - are fed either fortnightly (every two weeks) or every three weeks.
We keep computer records for every animal we own, so it is easy to note patterns and keep a track of who is feeding and what. From this we know that, for example, one of our Royal pythons - a female, who is still young, will only eat every two weeks so we have stopped offering her food every week. We also have a couple of snakes - namely one of our adult male Royal pythons and our adult male Dumerils boa - who periodically go off feed during winter, last year both for eight months, so over periods like this those who go off feed are tried every few weeks, and eventually they start eating again and are therefore usually fed weekly because they stop eating for so long in the year.
It's quite a process when you have a large number but the record-keeping definitely helps, although we know each of our snakes' tracks anyway.
Manhirwen - I would recommend trying your Royal every two weeks if he doesn't regularly accept food on a weekly basis. This way you will waste a lot less food and give him chance to work up an appetite! How old is s/he? What are you feeding? Boids generally have slower metabolisms than Colubrids and therefore often do better with more time inbetween feeds. (If you take note, Colubrids, in particular those pesty Corns haha, seem to excrete far more than they take in! While boids eat once and visit the toilet once!
) Also the food offered can make a big difference - some of our snakes will only eat mice even though they are big enough to eat rats, while some will only eat rats, and others eat whatever goes infront of them, especially if it has five fingers and is warm
