Awww, she's cute! Under normal circumstances I'd guess her to have about three weeks left, just based on my previous experience, but my own mare has had me thinking she was close for ages now so I certainly don't trust that anymore! I think your best bet is to assume she'll be close to her date, but be prepared no matter what!
Aww, it's so fun, with all these babies coming soon, I can't wait for them to start arriving and seeing the pictures of all the new lil foals!
Her belly looks like it's started to drop, but not very much yet, and overall she doesn't look very "big". It looks like her rt side teat is much, much fuller than her left? Might just be the angle of the picture. (is worth keeping an eye on in case you have a problem with mastitis) But see how her teats still face each other? When that deep center crease starts filling in between her left and right half, and the teats face downward or out from each other you will know she's getting pretty close. You can always start testing her milk if she's producing enough to test. I use CheckMare tests that my vet provided, but someone (I think it was CheyAut) mentioned you can just use pool test kits from Walmart!
Good luck and keep us posted!