When you get in high wind conditions the reason to use the storm sails is often to save the bigger sails. High winds can flog a sail to death is very short order, and flog they will, no matter what you do. And then you are really stuck unless you have a good sailmaker aboard. A storm trysail allows you to lash down the boom, which can be a man killer as it flails in high winds with the boat being tossed around by big waves. A storm jib on a forestay brings the sail inboard and gives better balance and certainly better shape than a rolled up genoa on a headstay and saves the genny for when the wind abates and you want to get to where you're going. Two small sails close to the center of effort are much better for boat balance than spreading them out as would happen with a rolled up genoa.
As to when to go to storm sails... it depends. You can't just give a wind speed. What kind of crew do you have at the time? With a crack, ocean savvy and large racing crew I hold on till the last possible moment. If I'm cruising shorthanded or out with inexperienced sailors I would shorten sail dramatically early in the game and certainly if it was getting late in the day. Also what heading to the wind are you making? I could see running before the wind with just a bit of a rolled up genoa showing but not trying to claw into it. Do you foresee heaving to? Nothing beats riding out a disturbance hove to with a Trysail and a Spitfire jib, sitting dry under the dodger with a cup of something hot in your hand.
It is impossible to give hard and fast answers. It depends on the boat, the crew (and the state of the crew - seasick? scared? hurt? exhausted? - or doing fine and eager for the challenge), the skipper (is he/she relaxed, confident and giving the rest of the crew confidence?) , the location (near a lee shore?). The list goes on. One of my favorite quotes comes from Will Rogers and fits the situation (and me)
"Good judgment comes from experience, and a lot of that comes from bad judgment."