Basically ... 12+ credits=100% scheduled semester award. 9-11=75%. 6-8=50%. 5 or fewer=25%. If the EFC is relatively high for Pell grant eligibility, there may not be any payment at less than 12 credits, or there may be a payment that does not coincide with the percentages above. If your EFC is >4000, you will want to check the schedules for the effect of less than full time on your payment.
If any part of the award is in the 2nd 100% of Pell for the year, there are other rules, as well.