Agree that the 24-28 weeks is standard. If it's on the earlier side, then there's a higher chance that they're not going to catch it unless they retest it. (I got my first three hour at a mere 14 weeks this time!) If it's later, then the only issue is that it could go uncontrolled longer. I'll get my second three hour test at almost 28 weeks.
*I have insulin resistance when not pg, which is why I get mine tested twice. Even with insulin resistance, my second test is still in the normal range for women without it.