There is no right time. There’s just right mindset.
When you are in relationship with her, there’s only two things to happen- She either enjoys sex with you or she doesn’t. Magnitude does not matter in that distinction.
Assume she enjoys sex. Now magnitude matters. The more she enjoys it, the more you enjoy it. Also the more she enjoys it the more she will try to please you. And the more she enjoys it, the greater the frequency of sex.
In such cases you should try whatever you can to discover new ways of pleasure. Try a sex toy, if it doesn’t make any difference, you were having great sex anyway. But if it somehow makes your sex life better, you can try some more and continue your journey to fulfilled sex life.
What if you refuse to use sex toys?
Well, refusing to introduce sex toys can indicate to her that you are not worried about her pleasure. Her enjoyment may decline which will impact your enjoyment and also frequency.
Now assume she doesn’t enjoy sex. She either does it to keep you happy or out of a sense of obligation.
If you introduce sex toy now, she might start to enjoy sex and you will too see positive benefits in your enjoyment and frequency. She now has sex out of a desire to have sex, rather than obligation or to keep you happy.
Inclusion of sex toys has potential to either enhance or hinder your enjoyment and frequency of sex. It can either increase hers or not affect it at all. If she is unhappy with the sex currently, this could change that easily. Not doing so could either leave things at status quo or hinder your sex life and the entire relationship. Relative probabilities are unknown to anyone but you would have the best guess.