How long can my alternator keep going?
 

I have 128,000 on my truck. I just got a new battery. I upgraded to a group 34/78 AGM battery.(had to get a new tray and hold down). Nothing wrong with the old battery, but they don't seem to last me long cuz all the acid spills out from offroading. Last time it died on me about 10 miles out on a dirt road. Luckily I was able to push start. I'm on the original alternator. seems to work fine. I would rather replace in my driveway than get stuck on some dirt road 50 miles from town and have to pay someone in hee haww hell to replace it.

I have a 300 watt sub. I don't blast it too often.

If I replace i am thinking about this one in 130 amp. Or factory.

92-05 Ranger 2.3/2.5/3.0/4.0L Alternator

Do you guys think it can keep going? Or should I replace it now? I'm surprised it has lasted this long.

Alternators rarely fail out right, fuses blow, fusible links burn out but alternators generally fail by degrees.
Alternator bearing going out is also an issue.

Alternators generate power thru 3 fields in the case, usually 1 field will fail when alternator gets old and that's when its time for a change.
Alternator can still run the truck and charge the battery with 2 working fields but at night you would notice dimming head lights at idle, because at low RPM and 1 failed field the alternators power output drops below 13.5volts with headlights on.

Put a voltmeter on the battery
Engine off should be 12.8v with new battery, 12.3v with 5 year old battery, 12.2v or below means get a new battery

Start engine, battery should now read 14.5v to 14.9v, under 15v, that's the alternator recharging battery after starter motor drained it
After a few minutes of running voltage should drop down to 13.6v, voltage regulator is working.
If you turn on all lights and heater fan to High, also leave doors open and crank up the tunes, lol
Voltage should drop and then come back up to 13.6v, all 3 fields are working