When I use maple syrup and jump to the secondary when the gravity is at about halfway, for me that has been about two weeks with the porter. Now for me I have a conical so primary and secondary are in the same unit and the only way to really define it is when I remove the collection ball it is to clean out all the trub and start the yeast collection. That is when I add the maple syrup.
With the syrup I would recommend placing the container it is in, in hot tap water to thin, same as what I do for mead. Don't bother to heat the syrup remember it is only a few degrees from becoming candy. The hot water for about 30 minutes and add it to the wort. Usually around 80 degrees, nothing hotter.
You will benefit from a good stir with the syrup, otherwise it will cool fast and sink to the bottom. Make sure everything is clean and sanitized because with stouts and porters lacto infection can happen, but for me I see that is as a good thing (that is another story). How much to add is up to you. I use 8 cups of grade AA syrup (half gallon) for five gallons of wort. You are not going to gain much in fermentable sugars from syrup as you would with sap but aging it for a few months, cold crash to settle out the yeast for a day or two, should yield a pretty good batch.