It just all depends. Basically, it's creating pages with new product.
I've been on CTs where I don't hear from the designer for 4-6 weeks at a time, she releases something(s) new and asks us to post it around, and then disappears again until her next new product is done.
I've been on CTs where the designer doesn't produce much product, but she has a very active forum/email list for her CTMs, and there is a constant chatter even when there are no new items in the shop to be worked with.
I've been on CTs where the designer has required 1 layout a week posted at several different galleries with those links posted back in her designer forum in addition to assigned newsletter sample layouts and/or blog postings.
I've been on CTs where the designer asks for feedback on much of the product she creates, and this feedback does very much influence the final kit or element being put on sale.
I've been on CTs where the designer told me to scrap on my own timetable and as long as I make a page from what I download and post it somewhere, there are no other requirements.
All the examples above are why it is SO IMPORTANT that you understand exactly what your designer will be asking of you when you became her CTM. You need to know how many layouts are required, where they are required to be posted, how active she needs you to be in public forums as well as her team forum/email list, and if she will be asking any additional obligations from you: blog postings, newsletter articles, contests and giveaways, etc.
The next time you see a designer post a call whose work you own, enjoy, and use on a regular basis... go for it! The worst that can happen is no, and while no sucks, it's not the end of the world.