Under what conditions does a Samyaksambuddha arise? Buddha arises. In the past there were several Buddhas. And it is said that in the future there will be at least one. 

My question is under what conditions does a Samyaksambuddha arise?
My question is under what conditions Buddha arises?


There's no set patterns. For example Buddha Gotama came during a relatively uneventful period where human life span was within normal range. However, for the future Buddha Metteyya to arise, per DN 26, life on earth must undergo some very drastic "downswing", and then some recovery, and finally thru some great upswing before He'll appear.