I put it on 8h, but it seem to be randomly chosen each time.

Could be a savedvariables issue. Also, note that for each item the last duration used is saved, so unless you've never posted an item this setting will be ignored.

Ok. Could we make it otherwise ? If a post duration is chosen, it supersede the last duration, and if no post duration is chosen, the last duration is used ?

I think it makes more sense this way, but if you want to clear your existing settings you can do it with /aux clear post