I have a 150hp 4stroke motor. I live in Iowa and we have pretty rough winters. So ig I'm asking when should you get your boat winterized. How many days of cold weather (also how cold is cold) until it's time? I'm just looking for a general theory that will protect my motor thank you

I live in central Minnesota and I winterize mine in late October and then start using it again in late April...so about 6 months.  Some people fish into November and start in March on the rivers here but I deer hunt in November and turkey hunt in April anyways.

There is no "time" when the outfit has to be winterized IF YOU KEEP WATER FROM FREEZING IN THE LOWER UNIT.  That is THE ISSUE.  Of course it is wise to change the plugs, lower unit lube (this is important because it gives you a chance to see if there is water in there which would indicate a seal problem, probably caused by fishing line damaging it), and fogging the engine (to prevent corrosion when you're no longer using it).  These other things are things that should be done with some regularity, but they don't have a deadline.  But you have to prevent water from freezing in the lower unit.  With most engines, maybe all, simply storing them with the lower unit vertical will do this.  My dealer said there is something  else on my Merc 60 that needs attention, too, and I haven't gotten the info on that yet, but usually it is as I wrote above.