What do you mean, "updated"?
- The record label, and /or the artist decide whether songs will be avaialble individually, or only as part of an album. They may change their minds, and make available (for 69 cents) a song that was only previously available as part of an album. Until any announcement is made by those people, no one know when - or if - that might happen.
- A song is available in the iTunes Store. It costs 69 cents. You buy it. End of story. The song is not "updated". Why would it be?
Ah! I have no idea.
Here in the UK, if I look in the store now, the list is dated 21st January. There is nothing to tell me when the previous update was, or when the next one will be, and of course, it may differ, according to the country you're in.
You may find that the only way is for you to work it out for yourself, by looking every few days until you can spot the pattern.