There are no side effects of long-term exposure listed anywhere by the books.
The spell Blink on page 219 of the PHB covers the effects of such for the duration of the spell, be that 1 round or forever. There are additional rules stating any side effects of the spell lasting longer by the book.
Certainly, a GM is free to design their own side effects, however.
Additionally, the Etherealness spell on page 236 of the PHB talks a little more about being on the Ethereal Plane for extended periods of time.
There might be interactions to keep in mind, such as wall of force effects on ethereal creatures. Also, see invisibility, truesight and other similar spells/effects are able to perceive creatures on the Ethereal Plane.
PHB page 301 also has this to say about the Ethereal Plane:
The Ethereal Plane is a misty, fog-bound dimension that is sometimes described as a great ocean. Its shores, called the Border Ethereal, overlap the Material Plane and the Inner Planes, so that every location on those planes has a corresponding location on the Ethereal Plane. Certain creatures can see into the Border Ethereal, and the see invisibility and true seeing spell grant that ability. Some magical effects also extend from the Material Plane into the Border Ethereal, particularly effects that use force energy such as forcecage and wall of force. The depths of the plane, the Deep Ethereal, are a region of swirling mists and colorful fogs.
Further indicating the effects of being on the plane itself. As far as the transition between the planes, there is no specific text in any of the books that mentions this. Largely this is not possible by written materials and since the situation was created by a GM created effect all side effects of the situation would also be left up to common since the the GM ruling on such.
References to the ethereal plane:
DMG pages: 43, 46, 48, 49 (encounters),
PHB Pages: 301, 219 (Blink spell), 238 (Etherealness spell)
I might also make reference to the multitude of creatures who are capable or actively enter and exit the ethereal plane all the time without ill effect as a supporting argument for no ill effect. Though, I strongly follow with "it is up to the GM" he may well rule that the effects on a person who is not designed for such might have some other effect not stated in any book.