After getting artery stents put in, how long should one be in hospital under observation?

Local practice. While many place stents and may not admit, most likely still do admit overnight for observation. It is unusual to stay more than one night unless other issues are present. Practices tend to follow local trends.
Overnight. Most people stay overnight after a procedure for stents. This can vary depending on other factors including chronic kidney disease, ckd, and other conditions - often referred to as comorbidities by physicians. Your doctor will assess your condition in deciding whether to keep you longer than overnight. All procedures have risks and any, even slight, complication could lengthen your recovery.
Same day. In an uncomplicated procedure, especially if your doctor uses a closure device on the artery, you can safely go home the same day.