Ticks
The game updates 20 times a second, so a game tick is 0.05 seconds. Redstone parts are timed in redstone ticks of 2 game ticks, 0.1 seconds: a repeater adds 1 to 4 of them, a comparator, a torch or an observer adds 1. Everything here assumes the game is keeping up; a server that lags, or a world sped up or slowed down with the /tick command, stretches or shrinks real time but not the tick counts. Checked against Java Edition 26.3.
Clocks
A repeater loop passes one short pulse round a ring of repeaters, so its period is the sum of their delays: five repeaters at 4 ticks make a 2-second clock. Start it with a pulse shorter than the loop, or the whole ring stays on. A hopper clock passes items from one hopper to another and back; each item takes 8 game ticks to move, so each item adds 0.8 seconds to the period, and a full hopper of 320 items gives about 4 minutes 16 seconds. An observer clock is two observers watching each other, on for 2 game ticks and off for 2.
Hoppers
A hopper moves one item every 8 game ticks: 2.5 items a second, 150 a minute, 9,000 an hour. A chain of hoppers is no faster than one, so to move more, run lines side by side. A full chest of 1,728 items takes 11 minutes 31.2 seconds through one hopper.
Furnaces and fuel
A furnace takes 10 seconds to smelt an item. A smoker (food only) and a blast furnace (ores, raw metal and metal gear only) take 5 seconds, but burn fuel twice as fast, so a fuel item smelts the same number of items in all three: coal 8, a block of coal 80, a blaze rod 12, a bucket of lava 100, a dried kelp block 20, a log or a plank 1.5. Part of the last fuel item is wasted when the batch is not a multiple of what it smelts, so a batch of 10 items needs 2 coal. Fuel is only used while there is something to smelt.
Bedrock Edition
Bedrock also runs 20 ticks a second, and its repeaters, hoppers and furnaces take the same times, so the tick, hopper and furnace numbers carry over. Its redstone schedules updates differently, though, so a clock built there can run a tick or so off what Java gives, and some Java designs do not work in Bedrock at all.
Questions
How many seconds is a redstone tick?
0.1 seconds: 2 game ticks of 0.05 seconds each. A repeater on its longest setting, 4 redstone ticks, is 0.4 seconds.
How many repeaters for a 1-minute delay?
A minute is 600 redstone ticks, which would take 150 repeaters at 4 ticks. A hopper clock does it with 75 items, or chain a few clocks. Put 60 into the period box under Clocks to see the build.
How many items does a hopper move in an hour?
9,000, at 2.5 items a second. If a farm makes more than that, split its output into more than one hopper line.
How much coal do I need to smelt a stack?
8 coal smelts 64 items exactly, in a furnace, smoker or blast furnace. A stack of 64 items in a furnace takes 10 minutes 40 seconds; in a blast furnace or smoker, 5 minutes 20 seconds.
Does it save anything?
No. Nothing is stored and nothing is sent anywhere. What you type is kept in the page's address, so a copied link opens the same numbers.