This is what I did. I set up a schedule so that every day, each room was cleaned to showing-ready level. For me it looked like this:
Early morning before DS got up: upstairs bathroom, dust (upstairs and downstairs alternated every couple days, didn't dust every day), and sweep/mop (kitchen/up bathroom, entry/down bathroom, then skip a day).
After DS woke up: bedroom, playroom, then vacuum (the entire upstairs, every day, downstairs every couple days since we weren't down there much, DS LOVES to help vacuum so that helps), then I also would go outside and make sure our curb appeal was still ok and all.
While DS was napping (I sleep with him but generally wake up before him): take out trash, then downstairs bathroom and two downstairs bedrooms (one had nothing much in it, the other was DH's office).
Before bed (while DH was putting DS to sleep): family room downstairs, entry, living room, dining room, kitchen.
This sounds like a LOT of cleaning and it was. But my house was always fabulous looking. I also wrote out a quick-cleaning list so that when I got that call for a showing in an hour I knew just what to do (eg. make sure dishes are washed and put away. make bed. wipe spots off bathroom mirror. etc.)
But the biggest thing I can probably recommend is to price your house on the low side for the market in your area so that it goes fast and you don't have to do this too long.

I wish you luck in selling.