There are just tons of variables in play. XD
"safe" is a subjective thing - if your hair is healthy, mostly, is what matters. You can highlight every three months, and if between highlights you never use heat or any other chemicals, DC, pre-oil and do PTs your hair will be decent anyway.
But if in between the 3-months highlights you always use heat to dry, sometimes you straighten, and the only thing you do is coconut oil treatments, it won't be very healthy.
But if you highlight every 6 months, and in the meanwhile you use heat sparingly, and you weekly DC, and -- see how many variables?
I would say 6 months highlights are perfectly fine, on my head. But I almost never use heat on my hair, I DC weekly, and so on. Consider your hair condition based on your routine, and decide.
[I]A 2C, wishfully thinking of being 3a. Sometimes I get there. CG since October 2013, Eurocurly (just for a few months more!), completely low-porosity now. SBL, wanting elbow length.
CW-LI: Garnier Ultra Doux (Cocoa+Coconut Oil/Mango+Tiaré) or my own products.
DC: Biofficina Toscana Maschera Nutriente Protettiva, or my own product.
Homemade FSG, L'Oreal Styling Gel.
I think it depends on your hair and what type of highlights you do. I usually highlight every other haircut (10-12 weeks.) I had medical issues last year and lost a ton of hair as a result. So, I stopped the highlights for a while, just in case they contributed to hair loss (for me they don't.) I'm getting them done again on Wed.
My hair is low porosity and doesn't seem to sustain too much, if any, damage from the process. I doubt that my highlights are on more then 5% of my hair. I wet, use a rinse out and style everyday--and use a professional dryer to diffuse. For me, it's protein that matters. My hair LOVES protein. I do at least one PT a week. I do use some really rich products in minute amounts as my stylers and never have to DT.
3a (Corkicelli), highlighted, fine, low porosity