It's always good to remember the legal boundaries when considering a monthly arrangement.

This site has a good compilation of common myths that everyone should be aware of: http://www.aacriminallaw.com/prostitution-common-myths-1.php

I do not advertise such arrangements on my site though.  These are agreed upon with each gentleman.
some are specific days of the week, like every sunday, for example.  Others are a little more flexible, once a week or once every other week.   They are custom made to fit the gents needs.   All are prepaid arrangements.  The gent gets a discount and I am paid in advance for a months worth of visits.  We both benefit from that arrangement.  Obviously, an arrangement you only want to do with those you have history with and know well.

Is it going to be exclusive?  then you are into sugar daddy territory and thats something I have no experience with.  But I have had one such "regular" relationships where the classic time-based arrangement was replaced with more of a friends-with-benefits kind of thing, but this was long after the classic arrangement had morphed into a close friendship and high level of mutual trust.  An understanding was reached on a monthly "allowance" and semi-monthly deposits were made to an account of hers by which she could access and use as she chose.  Additionally there were occasional gifts or shopping trips.  We informally agreed that there would be from x-y dates each month and these were almost always dinner dates and a couple of them each month were overnights (always at my hotel).   There were also occasional weekend trips together.  eventually it ended because I think both parties started feeling taken for granted whenever things did not measure up to prior experiences. For example, she might not be available when desired or the level of generosity by me was lower than usual.   For me, jealousy eventually set in and for her my feeling that I should be a higher priority became tiresome, so it eventually ended.  

It comes down to whether the lady sees value in the relationship itself and feels "time of the clock" so to speak is desired with the gentleman, or whether she still sees it as strictly within client/provider boundaries.  If the later, be VERY confident that the guy does not see it otherwise or hopes for more, or you risk an eventually stalker sort of threat when things get out of hand and you need to end it.  I know of several similar situations where things ended very badly and I am sure your provider colleagues all have horror stories to share as well.