Hi mate,
It most certainly will not go off. The cask will further mature any spirit added to the cask. The longer you leave it the better it gets. It's a test of your patience. Because the ratio of oak to spirit is much smaller than a full size barrel it will mature any spirit you put in the cask about 3 to 4 times faster than a full size barrel. The downside to this is the rate of evaporation will increase in proportion. It you intend to leave the whisky in the cask for a considerable period of time you really need to store the cask in a shed or a garage to keep it in a cooler environment. I would consider more than 6 months as a good time of thumb.
In the attached photo the bottle on the left is the retained sample of what I put in the cask. The bottle on the right is what came out the cask after 5 months. The cask was seasoned with Buckfast. The change in character of the whisky surprised even me. Deelish.