Depends on the mare and the test. I have one that will never ever pee in front of me,she's my alpha mare, so I duct tape a sanitary napkin just below her vulva and leave it for a few hours. This method works perfect for that mare. Then I have ones that ALWAYS pee when I come into the barn in the morning, so I just use a syringe to suck up the pee off the barn floor immediately. That works great for those mares. And I have one that MUST pee in a clean stall when she first gets in it. Her I can collect in a cup as I am right there when she goes in.
As to when, if it is a weefoal38 test, I wait until day 50. for a weefoal120 I like to wait until day 124 or so. If you have the patience I would wait and do the 120 day test as I find it has much less false negative results than the 38 day test.
Hope this helps some.
amanda