When should I take guitar lessons?
Most professions require some degree of continuing education. Doctors, schoolteachers, lawyers and so on. It is a rare musician that has the degree of honesty, self-awareness, work ethic, to sustain a lifelong musical undertaking without regression - becoming a hack. Worst of all, unknowingly.
Of good players, one occasionally hears 'they've never had lesson in their life'. I don't know any case where that is true. If not formal tuition, then certainly good role models and guidance. This is simply a matter of how we define 'lesson' or 'tuition'.
How often should I take a guitar lesson?
The clichéd 30 minutes a week is only recommended for beginners and young students. More mature people – teenagers to adult – should only come when they have made some progress with the previous work. Outside of a tertiary institution, an hour a fortnight can be better than a half hour per week.