The typical life span of chemicals is 1-2 years in most cases, longer if it can be frozen without a frequent rise and fall in temperature.
If you have a steady temp. then you should be fine as long as the bottles are capped and stored in a dark place. If not in the dark, at the very least in amber or blue glass bottles to prevent the the dreaded plastic "flavor" from taking over.
Natural ingredients are going to spoil multiple times faster than anything created by man. It's the same way with food which is preservative free. So if you're using naturally extracted juices or flavoring, it may or may not last more than a few months to a year. Artificial flavoring should definitely last a year unless it's contaminated from the get-go.