I will like to know, for how long the crust should stay Crackly/crisp?
My sour bread lose is crisp crust after 12 hours, Is this supposed to happen?
Yes, this is supposed to happen. The crust is drier than the crumb, so with time some water will migrate to the crust softening it. It is also possible that the crust will absorb some moisture from air.
Ok and How can I get the crust stay crispy for longer time?
heater, sunshine, oven, dry air, eventually the bread will dry out completely.
What I used to use on regular bread and now use on sour dough bread is either milk or butter.
The difference is that milk tends to make the crust chewier, where as butter will make it crusty.
Either the milk or butter have to be brushed on just after you have baked the loaf.
This tends to be a personal choice.
I just run the bread quickly under the faucet (not the cut side) and "rebake" in a preheated 300 degree oven for about 10 to 15 minutes.
I will try some of the suggestions you offered