how often do you have to login?

[Vahlnir]

As far as your island goes, you won't lose anything. Not even your captured creatures or crops you might have been growing at the time. All of it will be there when you get back, regardless of how long you've been gone. If you are looking for constant uptime though for progress, once or twice a week to check it all and get orders going for the next cycle would probably be sufficient.