Not as trivial if you don't want to use keys as @Cairyn already wrote. I suspect there is some fancy math to make this awesome 🙂
There are few problems here if I assume correctly what you are after:
1. No keyframes
2. Object has to eventually stop it's motion which is driven parametrically (by time). That assumes that after some duration movement should stop and time value which is a "driver" has to be preserved otherwise the time will simply keep moving the object
3. Gradual slowdown. This can be done by remapping duration to output multiplier
I attached a scene that does this, hope this is what you are after, couldn't think of anything better.
In memory node you set duration. For more complex decaying with more options you can use spline mapper for multiplier but this involves more work or maybe different approach
decay.c4d