
Quoting
flyingron
You've got two issues here.
First, how long you hold it has squat to do with anything. If the original investment is separate property and you don't do anything to it as part of your married estate (that the appreciation is due only to market pressures), then the appreciation is passive and also seperate property. If you bought GOOGLE at $500 before you were married and sell now at $1000, that is all your separate property (of course there's capital gains to consider). If you had stock in a company you worked for while married, that would possibly be a different story.
Now once you sell the asset, you have to be very careful what you do with the proceeds. If you do any comingling of those proceeds with your marital funds, they now become marital property. If you intend to buy and sell things that started as seperate assets and wish them to remain so, you need to keep those isolated.