Hmm.. I made whipped ganache for a filling the other day and I left it out for two or three days. I think it is ok that long, but after that or once it's cut open, it probably does need refrigeration. I hope someone else will let us know if that's not the case.
I would just use the same amount of white chocolate as you do milk chocolate or bittersweet chocolate for the ganache recipe. For whipped ganache do a 1:1 ratio of chocolate and heavy cream, add about 2T butter to retain the shininess of it, and 2T Karo syrup. Chop chocolate up small and use double boiler to melt. After melting the chocolate and butter, mix in cream and other ingredients til smooth. Take off heat and stir in a teaspoon vanilla.
To whip it, chill first for about an hour in the fridge. Some recipes recommend chilling the bowl and beaters, too. Whip for 2-3 minutes til it triples in volume and lightens in color (if you're doing dark chocolate, it won't change color with white probably).
If you are making ganache as a glaze, I'd do about 2:1 chocolate to cream or close to it, along with same quantities of the other ingredients.