Many providers advertise monthly arrangements on their website. Some are exclusive, others not.
Many more of us are also sugar babies in our personal lives, kept separate from escorting... For me, I seek out actual relationships from my benefactors (girlfriend status), whether exclusive or not, not allowances or P4P. I can do that escorting. For me, sugaring is much more personal and intimate. It IS a relationship, and there's normally a mentoring/networking component, too. That's just me, though; we all have different approaches towards sugar dating.
If you'd like to set up a monthly arrangement with a provider you already know, and she doesn't mention it on her site, I'd make sure you're REALLY a regular before even asking her. Many clients make the mistake of thinking that they're contributing more to an escort's overall income than they actually are, and they become entitled and expect more than they're ever going to get. Do you see her every other month, once a month, or once a week? The more you visit, the more she likely values your company. So if you only see her every so often, and are asking for a discount, that might piss her off. But if you see her frequently (twice a month, EVERY month, or more), then it's a topic you might broach, but with care....
I'd be careful to not come across as wanting a discount, or wanting to date her for free... Instead, make it sound like you'd rather prepay in full for your month's visits, rather than as you go, and would appreciate implementing more of a social component to your dates, and see what number she throws out there. Leave the ball in her court, so she can decide what a fair number is to her, then take it from there.
Good luck!