Hi:I noticed that gold and items in the crates respawn after sometime,how much time needs to pass for this to happen,thankyou

It varies from area to area. For example, New Sorpigal map respawns after 6 months (crates & monsters both).
Unfortunately I know exact respawn rates for M&M7 but not for M&M6, so I cannot help you more :)

Most outdoor locations in MM6 will respawn in 1-2 years of game time, so you probably won't see too many respawns over the course of the game. I'm not sure if indoor zones respawn, or if they do, what sort of timers they're on.
In MM3-5, I've heard that if you don't destroy the monster lairs on the overworld, the monsters will respawn, but I've never seen that happen. Outside of that, I don't believe anything respawns.

They do. All areas respawn.

If you're asking about MM6, then...
According to this magnificent guide, MOST dungeons respawn 2 years after you first entered them. Also, it lists reset timers for every outdoors area. Seems the tougher areas are quicker to respaws, with some exceptions.