I've done a lot of DXM, though not anymore, and I can definately help you out.
It'll take a few hours more than HBr to kick in, the comeup is drawn out and the peak and whole trip are extended. I'd say 3-6 hours in is the peak, depending on your personal metabolism.
However, some don't like this. There's an easy way to break down the polymer bond.
Take your delsym, add an equal amount of lemon juice. Leave for 30-60min, stirring occassionally, but vigourously.
That should break down 50% of the polisterix, giving you a faster comeup but also a longer trip from the 50% not broken down. 18 hours is enough, assuredly, to get at least 90 percent broken down.