If she was in season and he was viable then it only takes a few minutes, seconds for actual copulation to occur. Just because one didnt see the event doesnt mean that it didnt happen. I am sure they werent watched every minute of the day for entire weekend.
If she is a maiden mare (never had a foal) then she will carry higher than a seasoned broodmare and may simply look like she has gained weight but only in the middle. The only 100% for sure way of telling if she is pregnant is a Vet check. He/she will palpate her and can even tell you about how along she is.
You can also purchase a pregnancy test (fairly exspensive) but at this stage its probably unwarrented, wait on time alone, and in many cases you can feel/see for abdominal movement. I'm not talking about a flicking of a fly but actualy body part moving along the uterin wall bulging out the sides. The best way to feel for such, foal movement that is) is by holding your hand under her belly going diagonaly under from her stifle towards the udder. Hold with mild pressure. If the foal is active then you will feel it, if the foal is not then you wont. Check again at another time. (be sure that the mare doesnt kick your head off so be safe when doing this) In my past experience feeding time seems to induce activity in utero foals. But then again one has to measure the temperment of the mare for this can also be a time when she doesnt want to be bothered by a human feeling her in a tender area. Take caution. Has she showed regualr heat cycles after the rondevue (sp?) with the stud? This is not 100% conclusive but helpful. (some mares will show false heat cycles even if they are indeed pregnant. I had a mare that would do this. She would be pregnant but still semi responsive to stimuli, the only thing she would not do was break down all to gether with stud teaser.
At 335 she should have dropped considerably in the belly. (foal getting in position for birth) even in a maiden mare. Its not unusual for mares (esp maidens) to not bag up until the wee hours before delivery, some will bag up then dry up some then bag up again. They all (maidens not as much as seasoned mares) relax in the vulva area and the croup will become flaccid and jello like (I call it jello butt). Some will pace more often, some will paw the ground and will do the same impending delivery. Some do nothing then show up with a foal at thier side the next morning with a nice full udder. Some fill up weeks ahead and drip colostrum. Each mare is different and of thier own. IT is not unusual fore a maiden mare to deliver late even up to 360 days. I say get a Vet out and get the mare checked. Its a little late for her regime of pregnant mare vaccinations. Consult with Vet at time of Palpation.
She has definatly gained some weight (she is wider in her chest and in the rear area based on the top part of her rump.) She looks just fleshy but if she is maiden its hard to telll with a front shot. Got a profile (side) shot of her????