It depends on the company and the value being provided. I have some companies that send me a daily email and I happily receive it. Those are usually short bites of easily digestible content. If you send me a book every day, I am not going to read it.
If you are trying to sell me something every day, I am not going to read it. If your emails are only ever trying to sell me something, I probably won't read them.
But if you have great tips, insights, how-tos, great stories and I find value in it 80% of the time I am ok with whatever frequency you provide.
I typically recommend that people send at the frequency that they can consistently provide value at. To stay top of mind, you need to be doing that once a week. Monthly is likely not enough for people to remember you.
I am comfortable with once a week, my inbox gets filled with way too many newsletters and I probably read less than 10% of them.
I only open what I need or want to read at that given time.
I agree with Dawn, the most important thing is to bring value to the receiver; Personalization is equally important to consider with the frequency.