I added 1.87 lb of maple syrup to a big RIS. It's a little over a year now and still has a noticeable maple flavor. I added it all to secondary. However, I didn't wait for fermentation to completely end. I waited till it was noticeably slowing down by a fair amount and the racked to secondary with the maple syrup. I'm happy with the results. I want to change the recipe a bit, but it's the malt bill. The amount and way of adding the syrup with stay the same.
I would definitely cut back from that amount if you're planning on drinking it fresh. While I enjoyed the maple flavor, fresh it was a touch too strong. After a year it's really hit it's stride, but I was planning on sitting on this for at least a year anyway.