If the toners aren’t lasting more than about 4-6 shampoos, it might be your hair care regime that’s the problem.
Are you using a purple shampoo or conditioner weekly to help neutralise the yellow tones?
What usual shampoo do you use? Some shampoos contain cheap bulking agents that are too alkaline and can strip the colour. So for anyone who isn’t a natural blonde, they need to accept that it requires a lot of regular maintenance to keep it looking good.
Are you using straighteners on it? They need to be set on the lower setting, no higher than 160°. You might be surprised at how much colour comes out when you use heated tools on the hair.