Not sure how it works technically - but you can keep having highlights over and it will cover up your roots without making your hair "two tone"... I think the hair dresser puts most of the bleach/tint on the top of the hair and less at the bottom
I have a similar colour naturally and have been having highlights for years without any problems
Hair is in pretty good condition but I try and do a deep conditioning treatment once a week, because the bleach can be drying.
The last few times, I've started having tints as well as the bleach (so there's my natural colour, the very light bleach colour and a sort of light blonde tint). The three colours look more natural and mean I can go for a bit longer in between visits and the tint is less damaging than the peroxide bleach
I'd say you get highlights done every 6-8 weeks, your hairdresser should be able to weave out the already coloured parts when putting in the foils/meches and only put the colour where needed. TS is right though, having 2 or 3 shades does look more natural.
All permanent tints have peroxide, as that's the developer - the bleach is what will do the damage and make your hair very dry if it becomes over processed. Use intensive conditioners to help combat it. Some salons use 'lightening systems' instead of the harsher bleaches. These are less damaging, but still have a high lift effect.