Profile images and rate limits


Hi, new to Twitter development and trying to make sure Im doing this right.
We have a section on a site that highlights a top 120 type list.
For the display the profile pic is shown along with a link for the individuals.

Got everything working where I can make the call get the data for the profile pic etc…
The rate limit thing is what I need help figuring out.

The top 120 means 120 calls made out of 180.
So I have it calling once and saving the data to a database.

Now does that 180 reset every 15 minutes.

The list is always evolving so I am just going to set up a cron to update the images but Im trying to figure out often to set it to run. Thanks for any guidance.