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The battery will stop being charged when the battery is at 100%, so there's no risk on leaving your MacBook plugged in. Furthermore, a lot of users use their MacBooks Pro plugged in all the time. You only have to unplug it at least once a month to keep your battery life.
Apart from that, you can unplug it and make cycles as usual. For more information about your battery, see > http://www.apple.com/batteries/notebooks.html
No, you can leave it plugged in as long as you like. Every Apple product with a battery for over a decade has had built in over charge protection. You cannot overcharge any Apple device.
Just use the battery regularly, like a couple of times a week, at least partially. Regular use is best for lithium cells, as is avoiding deep discharge/charge cycles. Partial discharges and partial charges are fine.
Basically, plug in and unplug as needed or conveneint to do so and do not fixate on battery levels too much.