how long would a computer last without virus protection?

As others have said, depends what you do with it - but it only takes one infected web site or one dodgy email and you could have a problem. Most attacks target windows based machines (more of them and more vulnerable) but there are viruses that attack mac machines as well. Linux and Unix machines are less vulnerable (mac os is a locked down version of Unix) but that's not going to help most people. Chromebooks are, I believe, the most secure and don't need specific anti-virus software - everything else does.
Microsoft Windows Defender is supposed to be pretty good these days and should be on by default - you might want to google how to check it is running to give yourself peace of mind.

I've been running this current computer without virus protection for 8 years and it runs fine. I stick to clean, established sites with it. Shopping, email, social media, this site right here. I have another computer I use for digging through the cyber mud.