I used 1.25 lbs on my first Dubbel attempt. Actually it was 1 lb. of clear (hard) candi and .25 lbs of table sugar. It ended up being a trippel (due to both color and ABV), and while it was a decent beer in its own right, it wasn't the Chimay Red that I was shooting for. In the final assessment, I felt that amount lightened it up a little too much. FWIW, next time I am going to raise my specialty grain bill a little for my partial-mash and I'm going to use darker candi sugar (probably syrup).
My advice for a 2-gal batch would be 1/2 lb of syrup. And make sure it has some color to it (at least D-45), or it probably is indistinguishable from table sugar.
"If you serve this beer to a Michelob Ultra drinker, he or she will cry." - from description of Northern Brewer's 115th Dream Imperial IPA extract kit.