I think I know what you mean and I'll say around 15mph, on the roads where bikes have full lane access and the speed limit is 25-35mph. No idea what the rest of your commute is like and no idea what kind of traffic or road conditions you encounter. I'd suggest to help get over your fear of riding on the road, spend some time riding on less-travelled roads and get a feel for what makes you feel relatively safe. Only you can tell when that happens. Aside from speed, visibility is a huge factor in relative safety. Wear bright clothing. Get front and rear lights and ride with them on all the time. Pay attention to your surroundings and your intuition. Before long it'll all be 2nd nature.