Variable duration in mogrt, step by step guide

You have the option to create protected regions in the timeline when using the Essential Graphics workspace (panel). This lets you change the duration of the "middle part" in Premiere Pro while keeping the timing of the move-in and move-out animation constant. Just set markers and then edit them.

If you want to be able to adjust the timing of the animations inside a MOGRT, you have to add a slider to a control layer and tie that slider value to the tMax part of an ease or linear interpolation method. I have created about a hundred layer animators that move layers, text, and graphics into position and then move them out based on the layer's in and out points. None of them have keyframes. Many of them are also converted to mogrt's. 

For example, if you added this expression to a text layer's position property and tied the timing to the value of a Move frame and a move frames, you would have an animated text layer that you could turn into a MOGRT and adjust the timing of the animation by simply moving some sliders.

The design goal: animate a text layer from the left side of the screen until the left edge of the layer is lined up with the left safe title line (10% of the comp width). Hold the text there as long as needed, then move the text down and off the screen. The speed of the move can be controlled by two sliders.

Here's the expression:

movIn = framesToTime(effect("Move In frames")("Slider"));
movOut = framesToTime(effect("Move Out Frames")("Slider"));
txt = sourceRectAtTime();
safTkt = thisComp.width/10;
t = time - inPoint;
rst = outPoint - inPoint - movOut;
end = outPoint - inPoint;
sPosX = txt.width - txt.left - txt.width*2;
txtheight = txt.height - txt.top - txt.height;
restPos = sPosX + txt.width + thisComp.width /10;
mov1 = easeIn(t, 0, movIn, [sPosX, value[1]], [restPos, value[1]]);
mov2 = easeOut(t, rst, end, [0, 0], [0, thisComp.height - value[1] + txtheight]);
mov1 + mov2

Add those two sliders and this Transform/Position expression to a text layer, put it anywhere on the screen, and it will move into the safe area boundary, then drop off the screen based on the layer in and out points. Add the sliders and some protected areas to a MOGRT, and you should have a graphic that you can make any length and control the speed of the move-in and move-out.

I could have written the expression with a few more variables to make it clearer, but maybe this will give you a general idea. You can see the Protected Areas in the Timeline.