From my own experience, I would suggest getting in touch as early as possible, after the previous year's recruitment is out of the way. I had made my prospective advisor aware, and we were able to find time to discuss ideas and possible projects, and to even find several possible funding sources.
Perhaps this differs in departments with a more formal application process, but I found that the actual approach to the advisor was the important part - after that point, they were glad to encourage me to apply, since our research were well-aligned.
It does depend on the advisor though - some might not be able to even begin to think about next year... Others (perhaps more senior, and less daunted by thinking about funding someone that far ahead), may be hoping people get in touch. I suggest you find out how the intakes work - we have year-round applications and admissions, but generally people are concentrated around February or October. There's nothing to stop and application to start at another time, however, so obviously doing a bit of homework may help here (if it's not just annual admission in one group).