Bongo Fury

I know there's no definitive answer to this question but on average how long should a leisure battery last?

Mine's about two and a half years old now and is struggling to provide the juice that it used to.

Indeed, how long is a piece of string? - so much depends on how well it's been treated and how often it's been flattened. 2.5 years is probably a bit low, but I've heard of them failing in much less time. The one I've just bought has a 4 year guarantee, so fingers crossed

To be honest.....and remember until now I have not had solar to keep my LB charged.... the previous 2 LB's lasted 2 yrs and 2 and a half years respectively....both with 2 yr Gtee's.....so not bothered....the latest is like Francophile's a 4Yr Gtee one which is topped up constantly during the day via a solar panel...so the 4 years....shouldn't be a problem...
Key with LB's...is how much and how many times you punish them with a heavy drain....

Recently got solar which helps a lot but previously the LB has had 2 major drains and has had to do some hard work.

Looks like it may be time to change it.

I bought a new 100Ah battery about 2 and a half years ago. It still works fine, and I , of course, use solar, but I've noticed that without any charging that the fridge now lasts around 18 plus hours max (found this out recently on a 18 hour ferry crossing), whereas when it was new I got 2 and a half days out of it.

Interesting, it was my fridge that alerted me too.
It does fine for about 10 hours and then flashes the not enough juice light.
The battery has been worked hard so I'm going to give it a rest and get a new one.

I never use my fridge unless I'm hooked up as worried it would drain the main battery. Are the leisure battery and main battery totally separate systems? And dose running the bongo charge up both? How long roughly could I run the fridge for without flattening the battery?

Too many questions haha

It depends on what has been fitted;
If you have a split charge relay then the alternator will charge both batteries as you drive,
If its a compression fridge then you should get 2+ days from a fully charged l/b.