Factorio will never go on sale. Wube has said many times that puting the game on sale would be a slap in the face to those who crowdfunded the game before it launched.

The best time to buy Factorio was during the crowdfunded campaign. The second best time is now.

As a consession there is the free demo you can play to see if you like the game.

looking at factorio and wondering if it would like to be on sale for me

Spoiler: it would not. Good game, though.