hmm...haven't had an MV long enough to comment. But I think 3 years is a 'reasonable' amount of time.

but....i say just go with it till it just doesn't hold charge..... if it ain't broke, nothing to fix!

 

John,
I kept mine on for just over three years and changed it a month ago. The original battery was working fine when I changed it and like you I kept it on a tether but I only ride my F4 on track days. During the three and a bit years I had the original battery on I accidentally let it go completely flat twice and both times revived it with my Optimate and it still made it this far no problem.
I changed it simply coz I didn't want to get caught out with a dodgy battery and have it ruin my day, so I figured a £40 investment every three or so years is well worth it. I also took the opportunity to fit a slightly higher output (capacity) battery simply coz I always run the fans after killing the engine and they do run for a while each time.
After changing the battery I thought the bike fired up more urgently although I can't be sure if that was my imagination or not.
I'd say that (possibly) after three and a bit years and letting it go completely flat twice that the original battery may have been on its way out.

 

I'd say with the Optimate you'd get pretty long life out of the battery -- definitely at least three years is my experience with harley's sealed battery (no optimate/charger). I let my 1 year old 2005 brute go flat twice. I had electrical wierdness recently after a 3 week non-riding holiday -- the mechanic blamed the relay box, but after getting the Optimate 2 weeks ago, the bike is back to it's normal reliable self. No more electrical gremlins! One of the things I think I've learned from this is the MV (maybe relay box?) is sensitive to a dying/low battery, and the bike will do strange things like lose all electrics momentarily then spring back to life while riding, lights/turn signals glow after the key is removed, fan doesn't come on, etc.

If I were you, I'd keep doing what you're doing with optimate, and if gremlins don't creep in, change in maybe 4th/5yr, or beyond if you're feeling lucky.

 

I have gotten 8 years from a properly initialized and maintained MF battery. The key is proper initial set-up with correct type charger (high voltage / low current after allowing acid to fully absorb into plates), and then not allowing to go totally flat too damn often. The OptiMate is an excellent rejuvenator when / if that happens.

I think you should expect at least 5 years if you do things right. That is, with a decent battery to start. The Yuasa from Japan is pretty good. The US made Yuasa (from the merger with Exide...can you say Died Hard?) has a questionable quality reputation.

 

My 2001 model still has the original battery. Sometimes the bike will go 4-6 weeks without being ridden (a crime against MV Agustadom.....I know) :later: and it will still fire up. I don't use a battery tender either. I reckon it's still got a couple of years life left in it.

Cheers,

James