I don't think there is any specific rule addressing this. You could just use the rules for command word activated items. Those aren't OGM as far as I know, so I can't go into detail here.
I think the main problem here is, that the effectiveness of the change would largely depend on the spell/effect bestowed. For example lets assume a swift action or standard action activation: A 10 rounds Freedom of Movement effect would most of the time be a lot better, if you could split the duration, since you could use it to escape for example 5 grapples. A 10 round deathward would still be better sometimes, but often it wouldn't change much, since you'd want to be protected each time on the opponents turn.