The longest I've been without security was back when I had no high speed internet (from year 1998-2006). Oddly enough, back when I had dial-up over two years ago, even though we had a firewall and everything and kept it up to date, viruses and spyware still found their ways into the system. In comes my first broadband connection that is still here, my DSL, and haven't seen a virus since. Spyware may still pop in, but now that I have a Linux box sitting in my basement filtering my internet for me using my own settings, nothing ever gets in now.
When I had dial-up, I only had one computer. That one computer died, and I took the hard drive out, which served as a back-up hard drive down the road after I had gotten DSL after a hard drive crashed and died. When I ran a scan of that computer before I placed it on my network, and this thing before it died had the most up to date stuff ever at the time and had a scan run 2 days before a crash, had about 8 viruses on it, 64 instances of spyware, and about 300 tracking cookies (holy crap). My older gaming computer which replaced my dead computer, and was my only computer in the house until I got DSL, that had internet on it as well, and again, had anti-spyware apps, anti-virus apps, and firewall. That computer only got 3 viruses in it, but had no spyware in it, and it only had 3 or 4 tracking cookies from advertising companies.
So the longest time I've had no security was when I was on dial-up (kind of ironic, but it's true!). The DSL brought loads of security, but yet more incoming ports are open on it than the dial-up.