Is it harmful to stay on tamoxifen for 10 years?

Maybe. Most clinical data has shown benefit of tamoxifen (tam) increasing up to five years. After 5 years it seems to level or possibly even worsen. There may be some cancer cells that not only become resistant but to which tam begins to act as an agonist. That being said there are still the ongoing atlas and atom trials that are trying to get more definite. Another option is ais after 5yrs of tam.
Yes. Tamoxifen is very useful for breast cancer patients for 5 years. Many studies have proven that after that duration the side effects including uterine cancer may increase.