I don't use a lot of mono, but in general I've found the cheaper the line, the less overall longevity. I change if my line doesn't feel slick/smooth anymore, makes a crunching noise when I press down on the spool, or knots/twists itself when I pull 8-10 feet off the spool.
Line conditioner can help tremendously, but that's an additional cost - sometimes it is cheaper just to re-spool. I use straight fluorocarbon on several setups and some of those lines are expensive, so line conditioner is a must to prevent throwing $30 of line away. With conditioner, I find those lines can last 6+ months in some cases.
As Wayfarer mentioned, inspecting the line after you get home (or prior to your next trip) and cutting a few feet off the contact end can prevent problems due to abrasion, nicks, kinks, etc.
Another trick - do this 2-3 days before your trip - tie your bait on (or just clip a heavy sinker), take about 10-12 feet out and place the rod over a balcony, staircase, etc. This will stretch out the contact end and help minimize the coiling, and should make your first cast a little easier.