Depends on the vet. Some vets in California, who do nothing all day but palp cows, are doing it at 24 days, sometimes less. Most of us are mortals and will palp 35 days plus, often 42 days. I won't certify a pregnancy until 60 days, since they can resorb them less than this and come up open...
"I won't certify a pregnancy until 60 days, since they can resorb them less than this and come up open... "
Actually the recognized standard in the industry is 90 days. Research has shown that there is a fair amount of pregnancy loss between 60 and 90 days. After 90 days, there is a 95% chance that the pregancy will go to term. If there is money riding on the diagnosis (such as with E.T. pregnancies) you better insist on a 90 days "safe in calf" standard.
The reason is after 60 days, it will abort, not resorb. This means you should see something--discharge, membranes etc so that you'll know that she aborted. For ET pregnancies, absolutely get a 90 day guarantee!