Get a programmable battery charger that performs at least five different levels of charging and tests the battery and gives a health report. I have been using an Optimate 6 nine step battery charger for a number of years. It performs tests when you hook up and plug it in, then tells you whether the battery will be healthy enough to charge and use, or not. Here is a link to buy one, well worth the money:
OptiMate 6 Select 9-Step 5 Amp Battery Charger for 12V Starter and Deep Cycle Batteries - Free Shipping: BatteryMart.com
I have switched to a Lithium Ion battery in my Vulcan 2000 and my older ZR1100. My ZX9R still uses an AGM battery.
First thing you do is get out your multi tester or volt ohmmeter, charge your battery with whatever charger you have until it says it is fully charged. Take it off the charger and put your leads across the terminals and take a reading. It should read, at a minimum, 12.4 volts. Assuming you got a good reading, start the bike, run it a few minutes, shut down. Test terminals of battery. If the voltage has dropped below 12 volts, you have a potential problem, and unless it reads something like 11.8 volts, you likely have a bad battery. To further test it, you can disconnect the leads from the motorcycle to the battery. Leave the battery off those leads and charge it again, and this time keep track of how long it takes to get that fully charged reading of at least 12.4 volts. If it charges up really fast from whatever low reading you had, the battery is toast for sure.
Another thing to remember is you need to run continuity tests when you have the clicking you mentioned, assuming the battery is good. First test for voltage drop across the battery terminals while starting the bike. Note the voltage drop. If it drops below 12 volts, depending on how far, the battery is your problem. If not, test between battery and the next connection, which will be the hot lead on your starter relay. Note voltage drop. Next, test across the terminals of the relay, note voltage drop. Next test from battery to hot lead from relay to the starter, then from battery to the starter. You will also test shorter routes such as the wire from the relay to the starter, etc.
Voltage drops tell you where you have a problem in your starting circuit. There are videos on Youtube demonstrating how to do these, on cars. Same for bikes. Some give specific voltage values, or you can find articles that give values, at least for cars. Again, bikes will be about the same.
Battery life? It depends on how diligent you are at maintaining a full charge. If you keep the bike on a trickle charger at least once a month for a few days each time, and the charger is a good quality charger, not one from Harbor Freight (which will French fry your battery eventually), the battery will last a minimum of 5 years. I had one last 12 years.
AGM batteries require special procedures for charging fully. You must use a charger like the Optimate I listed above to get it fully charged. Especially if you allow it to drain down too far or completely dead. Using a non-programmable, non-AGM charger will result in progressively lower and lower charge capacity of the AGM battery until it only will start the bike maybe a half dozen times before it is drained. Look for chargers, if you aren't buying the Optimate, that have special programs for AGM and gel batteries as well as flooded cell batteries.