Under the Homeowners Protection Act, you can request PMI cancellation in writing once your loan balance reaches 80% of the home's original value, and servicers must cancel it automatically at 78%. Waiting for the automatic threshold costs several extra months of premiums, so requesting it as soon as you hit 80% is worth the small effort.
If your home has appreciated, you may be able to drop PMI even earlier based on a new appraisal — many lenders allow removal once your current equity reaches 20–25%, regardless of the original schedule. The appraisal costs a few hundred dollars but often pays for itself within a year. Note that FHA loans handle mortgage insurance differently, and it often can't be removed without refinancing.