The SWD rules give no suggestion for the length of a round in a Mass Battle are there any rules or hints for how long this should be for a battle type and size anywhere?
Mass battles are very abstract and cinematic in nature, and the length of individual rounds can and should vary depending on the size and type of forces involved, and may also change from round to round depending on the situation and what each side is trying to acheive.
The example in SWD describes a planetary invasion where one turn lasts a couple of hours. I've also seen and run examples where turn lengths have been between 15 and 30 minutes. These tend to be where there are two smaller armies facing off against each other. The formula for working out how much ammo/power points characters use also hints at this. 3d6 shots for a ranged weapon, trebled for burst or full auto indicates an extended period of time.