They are rated for 1,000 cycles. Health is the biggest determining factor (and failure/swelling). Download coconut battery and run that. <80% health...replace. >80% health, leave as is unless you have a problem.
Have you looked for the app called battery monitor on the mac App Store? You can basically monitor your battery by knowing the number of cycle, the battery capacity percentage, the charge percentage and the time remaining for complete charge or complete empty battery. Really good tool in my opinion...
When it swells up and you can't use the trackpad, or preferably a little earlier. As others have mentioned, look at capacity rather than the number of cycles and coconut battery is a very useful app.