I've often retained a plug for 24 hours without any harm coming from it. Since I have a wide variety of plugs I've come to realize that "retain-ability" seems to revolve about the length and the diameter of the neck. Most commercial plugs have either too short a neck causing pressure on the anus or too wide a diameter neck. The length of the shaft and its diameter does not seem to be a factor for me (within reason). I've found that wearing a harness is nice, reducing the concern at work, but having a smaller one slip out while shopping was very interesting. :o
One thing I have found to be very exciting is wearing a dildo held in by an elastic thong. I go out on the front porch and, as I chat with the passersby, my body will expel the dildo. But the elastic stops it from coming out very far (about 3 inches, although it feels like more) and, as my body relaxes, the elastic forces it back in. The expelling is usually involuntary although I can force it but the feeling of my body fucking itself is rather amazing.