Minecraft wiki entry for Tips_and_tricks, associated text: Setting up an "auto-smelter" or few with hoppers to feed in items and remove output makes life much more convenient.
A single furnace smelting a stack of items will take 10 minutes 40 seconds, or more than half a Minecraft day.
Using a blast furnace or smoker for appropriate jobs will take half the time: 5 minutes 20 seconds for a stack.
Splitting up jobs into multiple batches to use two or more furnaces (perhaps of different types) can save a lot of time.
While a few items worth of wasted fuel hardly matters in the long run, it's worth knowing a few numbers that will come out even:
8 pieces of coal or charcoal smelts a stack of items, a stack of coal smelts 8 stacks.
16 dried kelp blocks smelts 5 stacks of items, a stack of the blocks will smelt 20 stacks.
16 blaze rods smelts 3 stacks, a stack of rods smelts 12 stacks.
4 blocks of coal smelts 5 stacks.
A stack of 16 buckets, filled with lava (they will no longer stack) smelts 25 stacks, which is just two stacks short of a full chest. (And one of those empty slots will receive the emptied buckets.)
Most wooden "leftovers" will match planks: 2 stacks smelt 3 stacks of items. The others will mostly match either sticks (4 stacks smelt 3 stacks), or wooden tools and weapons (1 smelts 1).
In Bedrock Edition, slabs burn as long as planks, so you can double your burning time by converting planks to slabs. This even beats turning the logs into charcoal.
For really big jobs: Smelting a full chest of stackable items (1,728 items) in a single furnace will take most of 5 hours real time, or almost 2½ hours in a smoker or blast furnace. The fuel required is one of:
18 lava buckets (with over a stack's worth of time left over)
22 coal blocks, or 216 pieces (3 stacks+24 pieces). The blocks save 18 pieces of coal, and the leftover time can smelt another half-stack (4 pieces' worth).
87 (1 stack+23) dried kelp blocks
144 (2 stacks+16) blaze rods