This is one of those speed-control things that turns out to be trickier than it seems like it should be. Although it doesn't address this case exactly, this article has some good background info on calculating the "area under the curve", which is what you need in this case:

http://www.motionscript.com/articles/speed-control.html
This expression does that--it will begin to slow down at 6 seconds, and will slow to a stop over a one-second period:

rate = 150;

beginStop = 6;

stopDur = 1;

if (time < beginStop){

time*rate;

}else if (time < beginStop + stopDur){

r = linear(time,beginStop,beginStop + stopDur,rate,0);

(beginStop + stopDur/2)*rate - (beginStop+stopDur - time)*r/2;

}else{

rate*(beginStop + stopDur/2);

}

Dan