PETG might not be the best for long term UV exposure.
According to Midland Plastics, PETG doesn't hold up well in outdoor applications, and is susceptible to UV degradation. Unless you have an outdoor or UV rated PETG product which will have additional chemicals added to it, it may breakdown more quickly than you would expect. (the link I used was : http://www.midlandplastics.com/srtd_petg.htm)
Polycarbonate on the other hand does hold up better in outdoor applications and is better suited for long term exposure to UV light. Most quads and RC products you see that have a nice slick looking shell on them, are made from Polycarbonate. That stuff is usually vacuum formed sheets. It does still breakdown but it is a much stronger material. The downside is that PC requires a higher nozzle temperature, than PETG. (The reprap wiki has more info: http://reprap.org/wiki/Polycarbonate)
That being said PETG is not going to dissolve in sunlight. It will likely last upto a year if not longer depending on usage. If this is your first quad you're making, then odds are you'll have a crash due to hardware failure before excessive UV degradation (at least thats been my experience with prototype aircraft).