From my experience, which has included up to 10+ hours at a time it is possible as long as...

1) Like Shy said you keep it well lubed.

2) You build up to it slowly and get adjusted to it. Jumping straight to long term could cause issues with adjustment, etc.

Honestly, the lub was the least of my issues when I was doing it. The larger issue was, until i built up a tolerance for it and my body adjusted, i would experience lower back pain, which would then prompt me to immediately remove the plug for the day.