lerp functions don't take a speed, they take a parametric representation of how much they should be in between your start (A) and end (B) parameters.
Of course, if you have a constant speed, you can figure out how long it should take you to go from A to B doing some simple math. If you're moving at X m/s, and you need to travel Y m, then you can get time by doing Y / X (if you can't remember this, just remember your units and solve for seconds).
Now in your code you have a time amount, a start position, and an end position. You're also going to have to keep track of elapsed time to give you your parametric (0..1) value to pass into your lerp function.
So if this is in a
Tick function somewhere:
currentMovementTime += deltaTime;
var t = currentMovementTime / totalCalculatedTime;
var currentPosition = Lerp( startPosition, endPosition, t );
One common thing to do is that if your current elapsed time is greater than your total calculated time, just clamp it (so your t is 1). That way you'll calculate your end position to be the actual end position you want to be at.
All that being said, your particular examples aren't terribly suited for lerping. For rotating a turret you probably just want to apply a constant rotational velocity while the user has a button held down. Likewise for acceleration. Generally speaking lerping is only really useful for things that you know take a fixed amount of time and aren't dependent on continuous user input.