The general consensus on this forum is every day or every other day.
I’ve seen quite a few threads of, notably, guys saying they haven’t been in touch with their girlfriends for 3-4 days or even a week, with most people replying:
“She’s lost interest”, “Move on”, “She would be in touch every day if she was into you”, etc...
This, I find, is a projection of their own relationships and their communication.
My GF and I don’t spend our time hooked on our phones talking to eachother day in day out... it’s tedious and boring, and in my opinion: useless.
If you think texting is a big enough part of relationships to warrant a break up if your SO goes a few days without being in touch, you’re either immature and/or have not much else going on in your life.
My two cents.