You can create construction lines of infinite length in your AutoCAD drawing with the AutoCAD XLINE command.
I will demonstrate the AutoCAD XLINE command in this post. I start by entering the XLINE command into AutoCAD.
AutoCAD then prompts me for the start point of the XLINE. Having selected the starting point AutoCAD will prompt for the through point. I can choose multiple through points if I want to place xlines in multiple directions. Below I placed two.
Now I can e.g. use the AutoCAD OFFSET command to offset the existing xlines. Below I used the AutoCAD OFFSET command twice, offsetting existing xlines by 50 mm outwards.