Also, it seems like the threshold for diminishing returns for weapon damage in correlation to durability is somewhere around 75%
Easiest way to track this is by keeping the weapon in your hand and checking your character profile on the website. Weapon damage is [(CurrentHP/MaxHP) / 2] + 0.5. So basically it does 50% to 100% of its damage based on its durability. Never goes below 50%; it just breaks and ceases to exist. It costs about 15,000 to fortify a weapon to 255 max HP, and if you think you can use it for a long time without losing it, it is probably worth the investment to do so. It would allow you you use the weapon about 5x as much without losing damage, and thus go 5x as long between repairs.
Approximately 2 hours I believe. You can look at last update timer below the followers box. If you already know the max durability of the weapon/armor you can use arms lore on it to see how badly damaged it is.
To add to this. The easiest way is to use an alt to gm arms lore. It doesn't take long (overnight razor macro)and they can tell what condition it is in , though it is a vague description.
Not so vague if you refer to this page http://www.uorenaissance.com/info/Arms_Lore. Please note the second notice for the max durability is currently not working but the first part works fine and can be deciphered using that chart.