Batteries will normally last btw 2 - 4 years, especially if it's the new generation gel-type maintenance-free batteries. When a battery is nearing the end of its usefulness, one would normally get symptons, the most obvious is during starting when the engine comes to life either almost immediately (an indication that the battery is still ok) or the engine cranks over quite slowly before it comes to life (battery almost discharged or dead). Another way to test the battery strength is to just honk the horn without starting up the engine. Loud honk means battery is still ok. Why replace something that doesn't need replacing?
As for battery tenders, if you plan to leave bike idle for more than a month & beyond, then you'll need one. Otherwise, save the money for something else....