RTF's Fill immediately on realization, or at specific times in the turn - including at the very beginning of a turn..

They would necessarily fill only once per turn.

Implementation would be a matter of the engine checking for all RTFs after any change in the board.

Were RTFs to be implemented, a logical option for designers would be an End-Turn Factory, which would fill at the end of the player's turn. I.e., the menu would offer “Standard, Real Time, and End-Turn Factories.”