It depends on whether the LPN school is properly accredited.
In my home state of California, LVN schools only need to be state-approved by the Bureau of Postsecondary Education. LVN schools don't necessarily need to be accredited in California; however, many are.
If your LPN school is state-approved only, the curriculum might not count for any credit hours whatsoever. Rather, the state-approval will only permit you to take NCLEX upon completion of the program. The disadvantage to state-approved schools is the possibility that RN bridge programs may want you to start from scratch.
If your LPN program is also accredited, the curriculum will often count as one year's worth of credit hours (30 or so units). Most RN bridge programs will accept LPNs as advanced-placement students if they graduated from accredited LPN schools. Your home state may have differing rules.