Radiation does not "stay,". Radiation is always coming from a source. Dependending on the case, this source might be outside of the body (e.g. Co-60 treatment), so immediately after treatment, there is no radiation. Solid sources might or might not stay in the body after treatment, radiochemicals will decay and/or excreted more or less rapidly.

In addition to Erik's, for the case of internal radiation therapy (brachytherapy), in which a radiation source (called seed) is implanted into the treatment site, can cause the body to give off small amounts of radiation for a short period of time. Patients who receive temporary implants often stay in the hospital while the implant is in place and may have limitations on visitors. Their bodily fluids are not radioactive. Once the implant is removed, their body is radiation-free.

Patients with permanent implants give off small doses of radiation as long as the radiation source is active – usually a few weeks or months. As with patients receiving temporary implants, the body fluids and personal items of patients with permanent implants are not radioactive. Because it is low-level, the radiation usually doesn’t travel much beyond the area being treated, so there’s little chance of exposing others to radiation. Still, to be on the safe side, patients may be advised to limit contact with small children and pregnant women.

For systemic radiation therapy, in which radiopharmaceuticals (radioactive drugs) are used to treat certain cancers such as thyroid, bone, and prostate, the radioactive elements such as iodine, strontium, samarium, and radium gradually decays at their half-life. They eventually leave the body over time through saliva, sweat, and urine.

Kind regards.

Depends on the cancer treatment, what type are you trying to know?

do you mean long time effects?

Yes, I agree. It depends on the type of treatment. For example radioactive iodine (131-I as Sodium Iodide) for over-active thyroid using moderate doses and can have a half-life of about a week or a bit less in the body. On the other hand, the same material in much larger amounts used for thyroid cancer may have a much shorter half-life of a day or so, and is excreted very quickly. Other radioactive materials (in the form of seeds or needles) may gradually decay in the body, or be removed once enough dose is given. X-Rays or gamma rays used for treatment do not reside in the body and are present only for as long as the machine emitting them is on. If you have a specific question a much more specific answer is possible.

I think the following link will be helpful

https://www.cancer.ca/en/cancer-information/diagnosis-and-treatment/radiation-therapy/side-effects-of-radiation-therapy/?region=on

Best Regards Sikandar Khan