I died in a Minecraft world and I left my stuff on the floor right when I died (I died in Lava), but some stuff didn't burn. So, when I left, my stuff was on the ground. What will happen to the dropped items? Will they despawn?
When you're the host and you leave the world, all activity will pause. Once you return, everything will be exactly how it was when you left the world. So, since your stuff did not land in the lava, it will still be where you left it. It will despawn the same as it would if you had never left the world.
Items on the ground in loaded chunks despawn after 5 minutes. In unloaded chunks, they remain 'frozen' indefinitely. On servers chunks load within 10-12 chunks about (160-192 blocks) from any player. In single player that largely depends on your render distance setting.
That means if you died in a single player world and respawned fairly far from the place of death, you can take your time traveling there, but once you're within ~200 blocks the clock starts ticking and you should retrieve your items ASAP. If you respawned relatively close, the chunks never unloaded, and the items will have long despawned before this answer is posted. On a server the situation is a bit worse as any other players loitering in that area will keep it loaded and the despawn clock ticking.