Many people will start to do about a 20-25% water change when the free ammonia hits about .8 - 1.0 to bring it back down. If you have a lot of life on the rocks I would start doing small water changes at around .5 or a bit less. On basically bare rock some people just let it go and don't do any water changes.
When recently cycling a new tank with 65 lbs. of Florida aquacultured rock my free ammonia peaked at about .4 and took about 11 days. I also use Seachems Stability whenever I'm starting a new tank.