That depends on how heavy and how high you are.
Who says it's going to slow down and descend? Do all that at FL100 and you'll be going up.
But assuming you're high and heavy, your rate of descent will be pretty gentle, reducing all the time until the plane finds its equilibrium altitude for want of a better description. If that point is higher than you expected it will stop descending if you don't touch the levers - regardless of what's in the ALT window. That's my guess anyway.