As a student with 40 hours and as yet no X-Country experience, would it even be wise to take a formation course? .......what would be a sensible number of hours/length of time to have been flying solo before undertaking such a course?
There is no magical hours figure, because everyone's skills in handling an aircraft are different.
The best guide is when you can fly (and everything else that goes with it) the aircraft without thinking about it, and it's as second nature as driving a car. Then you're coming close - but not until then.