Adult Bearded Dragon Shedding

How often do adult bearded dragons shed? My oldest is 6 years old and for the last month or so her color has been very dull especially on her head. I'm 99.9% sure she just needs to shed, but it's been hanging around for a while. My mom has been caring for her while I've been at college, and I just got her back a couple weeks ago so I really don't know her cycles yet.
Details:
Her tank is very large... not 100% on the dimensions.
Warmest spot is in the 90s (considering a warmer basking spot, but she stays on the cooler side of the tank anyways) and coolest is in the 70s.
10.0 Reptisun UVB tube
She is 18 inches long
She eats a mixture of kale, mustard greens, and turnip greens every day dusted with a calcium powder
Eats around 8 super worms at a time when in the mood (she ate them the last two days)
**disclaimer** I have dubia roaches being shipped to my house as we speak... There was a mess up with my last order, so I had to switch to super worms (my local pet store doesn't sell dubias) for about a month until I could switch back to the company I order from.
"Soaked" 3-4 times a week... in quotation marks because it only lasts about 5 or 10 minutes of her sitting in warm water and me dripping it over her and her mouth so she drinks

Adults tend to shed more in patches than younger dragons do. A hand here, part of the tail there. Sometimes they'll even shed single scales at a time. The sheds seem to take longer to complete and are less frequent as well based on what I've seen. A dull/gray head or face for an extended period isn't alarming in itself. It may just be taking a while for the shed to complete.