Sellers should start packing as soon as they think they might want to put their home on the market. That is, they should start weeding out all the personal property they no longer want or need. It makes no sense to spend time and money packing and moving these things.
Most homes need decluttering before they're sold. This is so they show at their best, which usually results in a faster sale at a higher price. Cluttered homes look smaller than they are. And, an unkempt look puts buyers off. Clearing out the clutter early is a good idea even if you aren't sure when you'll be moving.