Alternator Failure Frequency?

Yamaha 90 2s manual states minimum CCA of 380 amps and minimum 124 minutes reserve capacity. By 2002 standards that is a size 24 battery that weighs 50 pounds. I went with a 60 pound size 27 and ran my boat and 12V TM off of that one battery until the day when I ran the TM too much and the engine was slow to crank.

So I bought a 38 pound Cabelas AGM 22 with CCA of 493 and 90 minutes to run my trolling motor and have on some days run the boat off of it with no problems.

So now I'm considering replacing my 60 pound size 27 with a 38 pound AGM 22, knowing that its reserve capacity is 27% under the recommended reserve capacity because I have the second TM battery on board that should cover the other 34 minutes.

It is my understanding that the main reason you need reserve capacity is in the event your alternator dies the engine can still run off the battery long enough to get home. But without a voltmeter you wouldn't know the alternator died until its too late, and that might not be until the third day of a four day trip while you're farthest from the dock.

So with the real risk being an alternator failure I'm asking how often have people had their alternators fail, and how did you figure out you had a problem?