You should check the engine oil at least once a month and preferably more often. If your engine is burning oil or losing it through a slow leak, you want to know this ASAP so you can address the issue promptly.
It only takes a few minutes to check the oil level, though an accurate reading requires that the engine be turned off at least 5 minutes to give the oil that was being pumped around the engine time to drain back into the oil pan under the engine. A good approach is to check it on a regular schedule that includes checking the air pressure in your tires and other fluid levels.
You should be more concerned about the oil level than its appearance. An engine that burns a lot of oil may have worn valves, piston rings or other parts that need replacing. Even if you haven't noticed puddles of oil under your car, you could have an oil leak. In either case, a low oil level could ultimately mean there isn't enough to lubricate all engine parts that need it, increasing wear and tear. In addition, oil circulating around the engine helps cool moving parts, so not having enough can make an engine run hotter.
It is much better to detect a low oil level sooner than later because the longer the condition exists, the greater the chances expensive repairs will be needed. Let the oil level get low enough and moving engine parts can get so hot they fuse together.
Oil is amber-colored when it's fresh and quickly gets darker and eventually turns black. Though that can be an indication that it's time for an oil change, you should use time and distance as your main guidelines. Follow the manufacturer's recommended intervals for changing the oil and oil filter, but do it at least once a year no matter how many miles you drive.