How frequently should I check my bank account statements?

Besides what Good Nelly has said, you also need to make sure that no unidentified transaction is listed, as such a thing could be a sign of credit card fraud or identity theft.

Try to check bank account statements every month to make sure that that your financial records match with that of the bank’s.
Make sure you check the summary of the statement and the transaction description to be sure that the withdrawals, deposits and fees, if any, are correctly listed.

After 2-3 months? Are you going nuts? Make sure you’re checking it at least once in a month. This way you can verify whether the charges are accurate or not. Tally the statement with proper receipts and if you find any error, dispute it asap. But I would suggest you to check your account statement online once or twice a week to make sure your budget is on track.