I go by the rule of thumb that Grog stated. Plus if you have any kind of fever you should rest.

I had flu with similar symptoms to yours a few weeks ago and it really took the stuffing out of me. I wanted to get out and ride but I just didn't have the energy. Every other day I would get on my trainer and see how I felt. If I was not enjoying it after 15 minutes I would bag it and take a nap. By the end of the week I was feeling good enough on the trainer to spin for 30-40 minutes then I knew I was well enough to get back outside on the road.

Hope you are feeling better and are back on the bike soon!