How can there be one rule to fit all??
Different families, different needs, different ideas on what is necessary to stay in touch.
My son, (for example) detests talking to ANYONE on the phone, if he does ring, then I will need to do all the talking. With modern technology, he would know very quickly if I was unwell ,etc and then would come straight down to me. I do not need to chatter away meaninglessly to him on some sort of calendar schedule to know that I love him and he loves me.
One of my daughter usually contact me (or me her about one a fortnight), again, no contact means nothing wrong. Two other daughters I only chat on the phone to every few weeks, sometimes I ring them, sometimes they ring me. Again, we are close and do care about each other, but do not need telephone chat to prove it.
Last daughter lives close by and I see her once or twice a week. If there was any problem with me, she would be quickly in touch with her siblings
One month not speaking may be normal for some families, for others it may be too long - there cannot be any set rules.