An LED TV can be left on all the time if you want to leave it on.
I have an LED TV in my home office that I leave on all the time when I'm home and working in the home office.
I'm not always watching the LED TV but I like having it on for background noise as it helps me focus and I will sometimes take a break from working online and then turn my chair around to watch what is on TV.
An LED TV that is on for around 8 hours per day should last 6 years before it quits or goes dim.
However some LED TV's will fail due to the electronic circuit boards failing or capacitors failing in the TV before they go completely DIM.
My LED TV only uses at most $1.00 per month in electricity costs and around $12.00 per year and is well worth it to leave it on.