When to Design and Check requirements in scrum?
In scrum when we should design software architecture ?.
because software architecture can affect the time of develop and estimations
Do you think there should ever be a *specific* time for this in an agile way of working, or ought architecture to be emergent?
Software architecture is about design and design decisions. Thus, we should be talking about taking decisions to see the software architecture emerge throughout the Sprints. When these decisions are taken? In every Sprint. Each Sprint ends with a piece of working software (Done, Increment) which already has architectural decisions incorporated. Depending on your context (and what the PO has deemed as valuable), you can also end with documentation for software architecture as companion to the working software.
Of course, there is no problem with crafting an initial "big-overall-picture diagram" in the first hours of your first Sprint, but remember that a diagram is only a diagram ;) not a software architecture.