Waldo,
I have made "inverted sugar" as much as a gallon at a time and kept it up to around 1 month for the purpose of adding it to wine, but not as a sweetner of finished wine. If left on the shelf for long periods pending the temperature...it will spoil. I left a remaining portion in the cellar of 60 degrees, exceeding five weeks and had to dump it.
Here is the formula from Dr. Jack's site for making of "inverted sugar" if you don't have it.
<DT>Invert Sugar:
<DD>The product of the hydrolysis of sucrose, which is glucose and fructose. Yeast convert invert sugar more rapidly than sucrose, such as simple cane sugar, because they do not have to break the sucrose down into glucose and fructose themselves. Invert sugar can be made by dissolving two parts sugar into one part water, adding two teaspoons lemon juice per pound of sugar, bringing this almost to a boil, and holding it there for 30 minutes (NOT allowing it to boil). If not to be used immediately upon cooling, this can be poured into a sealable jar, sealed and cooled in the refrigerator. Invert sugar should NOT be used to sweeten finished wine as it will encourage refermentation. </DD>
<DD>Hope this helps..</DD>