2.5% concentration is 25mg/ml, so .5ml would be 12.5mg per dose.
My girls usually got .3ml of 22.7mg/ml concentration for UTIs, or 6.81mg per dose. My girls were about 750g, so small pigs, so about 9mg/kg per dose, and they would get it for at least two weeks.
If your piggy is a but over a kilogram, it would be an equivalent dosage, and I'd expect the second week to be required (one of my girls sometimes needed up to three weeks). If your piggy is a kilogram or under, it's a slightly higher dose, but not necessarily out of range and may have accounted for the shorter initial duration.
Keep an eye on appetite with antibiotics (any antibiotics), as they can cause some digestive upset (not just in piggies).