Lovely weather aye?
I have seen frames capped in two weeks, and I have seen frames uncapped after a month.. It will also depend somewhat on how thick your syrup is. 2/1 or thicker will take less time for them to dehydrate. I fed up until a week ago, but as you stated, I am considering adding a bit more to the hives that are still a bit light....
If I do, and they dont cap it in time, it will add extra moisture to the hive as a whole... In that situation, having an upper vent becomes more important!