Once an area respawns, everything resets, and anything in the chests is lost.As Zedd says, the best you can do is to find somewhere that does not respawn for 2 years, and the most convenient are probably either inside Goblinwatch dungeons or inside the Abandoned Temple of Baa, in New Sorpigal. The timer starts from the first time you enter these dungeons, so if you keep an eye on the calendar, you should be OK.
In MM7, you can store items permanently in the chests inside Harmondale Castle, and in MM8, you can go to the Adventurers' Guild and give them to NPCs that you don't want to take into your party.
In MM9, I think you can store unwanted items in the banks once you have completed the two quests that you get from Brynhildr the Money-wise in Thronheim.
