About 2 weeks is a good starting point for most ales, longer for higher gravity beers. You will want to check the gravity and then check it again a couple days later. If the gravity remains the same then it is safe to bottle. If it drops, then wait a couple more days until you get consecutive readings that are the same. You are correct, you want to keep opening the lid to the minimum. Be sure to keep your theif or turkey baster well sanitized when taking samples. Most people recommend tasting the sample rather than dumping it back in the fermenter to avoid contamination.