Hi Lawrence,
When you buy hits, you are buying exactly that number of hits. They never expire so they could last a year or 10 years or 2 days.
So to work out how long your hits will last you must calculate how often you hit the feed. Note that there is no point in hitting the feed more than once every 2 minutes because it doesn't update more often than that. Also note that you are wasting your hits if you hit the feed when markets are closed because the prices do not change at that time.
Cheers,
Sidd