What is the life expectancy of someone who has metastatic squamous cell carcinoma?

It depends: A low-grade squamous cell carcinoma of the mouth metastatic to one neck lymph node may get a cure. A low-grade lung cancer with bone metastases may kill in a few days by causing pneumonia, or be compatible with a few years' survival. In real life, no physician says, "you have thus-and-so long to live". I hope that you will cherish each day of life that you are given, living richly & loving well.

Statisics vs. n-of-1: Oncologists are good at giving prognostic life expectancy statistics for a population of patients with a specific cancer (with associated grade, stage, biomarkers, etc). However, we are not good (usually) at predicting the life expectancy of an individual patient. This is improving both in the the untreated patient (prognostic markers, etc) and after treatment but is still imperfect.

Uncertain: It is notoriously difficult to predict the life span in a given patient, especially without knowing all of the person's medical history.