I can go 6-7 weeks but that’s part due to the short cut needing doing. It doesn’t go too brassy as I use a good blue shampoo. Some toners last longer than others too, e.g. for me pinks last longer than lilacs Also, wash it as I frequently as possible el
See - my hair literally turns brassy within a few washes and I’m not sure why.
The attached pic is 2-3 weeks difference so I would say about 2 washes a week? It most lightning it just looks brown rather than blonde. And yes I have silver shampoos and masks etc but it never really works that well once the toner fades.
Mine goes brassy really easily too. I add a bit of purple dye to conditioner and apply it to dry hair before I wash it and that keeps it icy cool. Don't find purple shampoos, regardless of the brand, are nearly as effective.