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.
Approximately every 2 months, iTunes displays a link that says "69¢ Songs" on their main page (Music Page). They usually keep the link available for about 2 weeks. However, there is always a large selection of 69¢ songs available on iTunes. It's easy to find them. Open the iTunes app on your device. Click on the music tab at the bottom of the screen. Then type "69 cent songs" in the search box at the top of the page. The first block of results will be individual songs. To the far right of the list of songs, click the link that says "All Songs." As you scroll to the bottom of the page on the "All Songs" results, more songs will load after you reach the bottom of the page. It takes about 2 to 3 seconds for more songs to appear every time you reach the bottom of the page. I've downloaded hundreds of .69¢ songs.