How likely is it for endometrial cancer to metastasize?

It depends. The answer to this really depends on the stage, grade and histology of the endometrial cancer. For example, early stage, low grade adenocarcinomas have a low chance to metastasize whereas a higher stage, papillary serous type endometrial cancer has a higher risk of metastasizing.
Endometrial cancer. It depends on the type of endometrial cancer, the grade of endometrial cancer and the thoroughness of the initial surgery. While the most common type spreads only about 15% of the time other more aggressive types spread more often (40-50%).
Uterine CA Spread. Patients with grade 1 tumors ) have a low risk (<5%) of nodal involvement. Patients with grade 2 or 3 tumors have a 5% to 9% incidence of pelvic node involvement and a 4% incidence of positive para-aortic nodes. Patients with deep muscle invasion and high-grade tumors and/or intraperitoneal disease have a significant risk of nodal spread, 20% to 60% to pelvic nodes and 10% to 30% to aortic nodes.