First you need to know what each test really does.
The CEA blood test is a marker that can indicate active cancer, but there are a number of other things that can cause CEA to go up so it is not definitive. Usually they check CEA every 6 weeks when they flush your port (if you have one).
A CT scan, like an x-ray sees different densities of 4tissue inside you. Cancer tends to be denser than surounding tissue, but again there are a number of other things that can cause denser tissue, so a CT scan is also not definitive. I have been getting CT scans every 3 months, which is considered normal for patients with recent active cancers. Once you are a couple of years clean then they will expand the interval.
Colonoscopies actually look at the colon and can see cancer thus they are definitive for cancer in the colon but only see the colon. They are usually done at three year intervals. The real purpose of a colonoscopy is to find and remove precancerous polyps before they become cancer.
If the CEA is elevated you will likely have a CT ASAP. If a CT scan shows something that looks like it could be cancer, what happens will depend on where it is and how big it is. They may just monitor it with another CT scan in 3 months or they may try and biopsey it to get a better idea of what it is.
I hope this helps. If you have any questions about this let me know. Good luck!!