You're the boss, so whenever you want!
Indoors, it doesn't really matter. You could plant them when your current plants go into flower and try to tune in timing so you have a perpetual cycle.
Outdoors, I recommend starting them inside about a month before you can put them outside. That will give them time to grow to take on elements outside, as well as be big enough to get nibbled on by something and still survive. And, once they look like they have a month left, you can plant some more beans inside and stick them outside when you harvest your first batch!
I hope this helps.