Personally, when I wear my plug for too long I tend to get a slight lower back pain, not dissimilar to the one I get when I have my period. I am fairly sure this is my body's way of telling me, "Enough now." Because I have practice with wearing my plug, I already know, roughly, how long my body can tolerate it.
My advice would be to make sure you give your body plenty of opportunity to rest and also to be aware of any physical changes, such as lower back pain, the need to go to the toilet more often, or constipation, which may be indicators that your body is not tolerating the plug well.
Wearing a plug for an extended period of time shouldn't cause any long term issues. There is no medical evidence that wearing a plug or having regular pleasurable/pain-free anal sex causes long term issues. If it feels good and you are in tune with your body, then all is probably well.
Remember to always use plenty of lube when inserting your butt plug, remove it regularly to go to the bathroom and most importantly, listen to your body. (Get some anal sex toy tips in Anal Sex Toys 101.)
