analog

It's not healthy for an erection to last for long periods of time. When the blood is in your penis, causing it to be erect, it is not circulating- it's "sitting there" (for all intents and purposes). If you maintain a constant erection for a period of several hours or more, the tissue in the penis isn't getting fresh, oxygenated blood and starts becoming injured tissue. At 4 hours, the time period the erection pills suggest after which you consult a doctor, is when tissue injury can start resulting in tissue death. With repeatedly injured tissue, you stand the risk of scar tissue forming when the tissue finally begins to heal. If scar tissue develops, it can eventually lead to erectile dysfunctions and/or change the curvature of the penis.

The penis is not designed to be erect for many hours at a time, without any periods of relaxation in between.

However, if you're looking to keep up stimulation without constant manipulation, you might try an anal toy that vibrates and operates with a remote (this is, of course, if the vibration gets you going any more than the toy's insertion, in and of itself, does). The general rule is anything up to 4 hours is medically acceptable, though I would not say it's "healthy" to ride that limitation. That's considered a "maximum". Talk to your doctor about how long might be ok to do as an ongoing thing.