Is it best use pour or pendant/depuis for a fixed duration starting now? Or can I use either?

Depuis cannot be used for a timed event starting in the present and continuing into the future. It can only be used to express a duration of time that started in the past and still continues in the present, or that both started and stopped in the past.

 

Pendant can be used to describe something that happened in the past, or the future, with no relevance to the present time. Pour can only be used relating to the future. 

For your example, pendant or pour could be used, and you could say Je viens d'arriver. Je resterai pendant/pour une heure.