Id assumed she uses extensions and is only ever seen if her hair was dyed the day before.
This interview from 2018 says weekly dying
In an interview with Refinery29, Stefani's longtime stylist, Danilo Dixon, spilled the details about the upkeep of the singer and The Voice coach's signature style. "We color her hair every Monday before we film The Voice. Gwen's an Italian girl with wavy brown hair — if you look at the video for 'Cool,' we did her in her natural shade, maybe a little darker," he said. "I learned that I could get to a place of achieving the blond that I want, and the quality of hair that we need and want, by using a new product, as opposed to just bleach." Dixon added, "People are, like, 'That's not natural,' and I say, 'Of course it is — it's supernatural.'"
That's right: If you're as roots-averse and blond-obsessed as Stefani, the solution is weekly dye sessions without heavy-duty bleach. That shit is bananas. B-A-N-A-okay, we'll stop, sorry.