At 70F I wouldn’t let it sit more than 1 day mixed. Molasses and corn steep liquor are complete nutrition with calories, so pretty much anything you can imagine will start growing in there. At 40F, I would guess 3 days.

You’re putting food into the soil, so you want to do it in very low doses, otherwise you’ll disrupt the metabolic stability of the soil (a fancy way of saying, you can create a bloom of something followed by a crash). You want the plants to be regulating the metabolic stability of the soil, not spikes of fertilizer.

I would suggest the dilution rate to be about +200ppm for molasses and corn steep. So measure the TDS of your water, then mix molasses and corn steep into the water until the TDS goes up 200ppm for each one. These are just first shot approximations. Apply, observe and adjust.