I have a cheap ebay air cooled 800kw spindle. I do not run it 24/7 however 3-4 hour sessions are common. Gets warm but never uncomfortable to touch. I would not give it a second thought to run it longer. 3 years and still going strong.
I understand this line of thought however what is the point in automation if you cannot allow the machine to work on it's own? Failure is bound to happen whether you are there or not however after establishing some history of reliability I hope you can at least trust your limit switches to save your machine and only lose time, material, and maybe a cutter when things go south. Crap does happen though.