all the time. It's rare when I actually ride with someone.
I've trained for centuries alone, multi-day tours alone, and even trained for my Ironman all alone.

I find that since I started riding alone I don't mind it. In fact, riding with others is distracting to me. I feel pressured to keep up at a certain pace and worry that I'm slowing others down if they stay with me (or I feel like I'm slowing my pace to stay with them).
I guess since I spend all day talking at work (teacher), riding/training alone is *my* time- and I prefer the solitude.

Just make sure you're prepared to take care of you, your bike, and be aware. Carry your cell phone, ID (or Road ID), some cash and a tiny first aid kit. Always make sure someone has some idea where you're at and when to expect you home.