15 weeks pg Was taking 250mg Ampicillin every 6 hrs for Strep throat for 2 days. Now switch to 500mg Amoxicillin 2x/day. Took Amp at 1:30 when to take?

Doesn't matter. You can start the amoxicillin whenever you wish. These are extremely safe drugs: you could take ten times as much of either one, or both of them, without harm to you or your baby. Don't worry about it. Best wishes for quick success in treating your infection and for a healthy pregnancy and baby.