First ever perpetually late paying tenant - help/ideas

[Kuriakos Mellos]

Hi BP community!

After hitting a very solid stretch of on time payments across my portfolio over the last 3 years, I’ve run into an issue with one of my LTRs where I have a tenant who now pays late every month.  I’m not saying a day or two or five - but usually the third or last week of the month.  She does pay the late fee when she pays.  It’s a one year lease, we are in month 4 - still yet to pay for February - but more so worried that this will start dragging into a month late or more.  
Again up to this point been lucky with great tenants across my 8 units - so maybe I’m over thinking here, and she is pleasant in person and keeps her area generally cleanish.  

However the excuses she has (and I’m sure we could all write a book about them) are, how to say it kindly, laughable almost.

I usually get a text saying she will pay by a certain date, which passes and left her a voicemail today asking to talk - I am trying to be understanding and said let’s talk on the phone to discuss a plan, but at the same time - don’t want to keep someone around that is not going to follow their lease terms.

Questions:

How often is it OK to ping a tenant for late rent?  Every few days?

If I see this continuing do I offer a a cash for keys situation to just get her out?  

Maybe I’m overthinking but trying to get ahead of any issues - thanks!