How long should I allow my dogs to continue to mate?

Question
I own 2 chihuahuas and have waited for all their testing and my girls 4 heat cycle to breed them.  I also have a mentor with 30 years of experience but would like to get a second opinion.  I know my girl cycles every 7 months and have familiarized myself with her particular signs.  She seems quite typical in her outward signs, 7-8 days of bleeding, then she chews her tail for about 10 days during estrus I suspect (which she hasn't once chewed her tail this cycle) and then her swelling goes down over a few days.  In all the cycle lasts about the textbook 3 weeks.  However, this is the first time I have allowed my boy around her during her heat.  I let them "play" supervised on day 7 and 8 of her cycle but she would not stand and would actively sit down when he tried to mount her.  Also their playing was more aggressive then usual.  On day 9 of her heat she not only teased him by sticking her bottom in his face, she flagged, wagged her tail and we had a successful 30 minute tie.  Anyway, they have tied once a day every day since.  She is on day 16 of her heat so that is 8 20 minute or more ties (once a day per suggestion of my mentor since we didn't do progesterone testing, although if she doesn't catch this time I will do so next time)!  My mentor says to let them go ahead and mate for as long as she will allow him.  However she seems to be displaying more of the aggressive play that she did before they started mateing but as soon as he mounts her she flags and stands for him.  Also, over the past 2 days her vulva is less swollen then it has been (although still quite large). What do you suggest?  Since it's day 16 call it quits and see what happens or let them continue to mate as long as she is receptive to him?  I also want to be able to schedule the ultrasound 28 days after their last tie but it seems they will go on forever at this rate?

Answer
Hi Heather

She seems to have hit her stride.  You can continue to let them breed until she won't stand, but then you have a greater span to figure due dates.  If the swelling is starting to go down, she should be finishing.

If I was in your shoes, I would let them continue for the next day or two, just in case, especially if you didn't do progesterone testing.