Start at the cable car station of Hochplattenbahn.
Marquartstein -> Staffnalm 1h30min
Staffnalm -> Hochplatte 2h15min
Via the sledge slope up to the Staffnalm is an easy hike. Further on to Hochplatte is doable with little snow. The last meters from the saddle up to the peak are partly steep. You have to be an experienced winter hiker to do this!
Snow shoes from Staffnalm / mountain station onwards can be handy if there is much snow.
Option: from mountain station you can hike around Staffn (about 1h45min) with amazing views!