How do you know when it is time to replace your chain?
As a chain wears, the pins and plate holes wear, increasing the spacing between rollers. This concentrates load on the top gear tooth, rather than distributing it over all of the teeth around which the chain is wrapped. Over time, the gear teeth will become hook-shaped and the tooth valleys will lengthen. A new chain will skip and jump when running on worn cogs and vice versa.
Chains are cheaper than cogsets and chainrings, and to avoid having to change all of them at once, check your chain regularly and replace it as needed.
The easiest way to check your chain is to measure it. Since bicycle chains measure a half-inch between adjacent rivets, there should be exactly 12 inner and outer link pairs in one foot. Set one end of a ruler on a rivet edge, and look at the rivet 12 inches away. If its edge is 12-1⁄8 inch away, replace the chain. If it is 12-1⁄16 inch away, replace it if you use titanium or aluminum cogs or an 11-tooth cog.
There are also many chain elongation gauges on the market. Two stand out in their simplicity and accuracy, the ProGold and the Rohloff.
With the ProGold tool, brace the hooked end against a chain roller and push the long tooth into the chain. If it reaches "90 percent" on the gauge, consider replacing the chain. If it reaches the "100 percent" mark, definitely replace it.
With the Rohloff gauge, brace the hook end against a chain roller, and if its curved tooth falls completely into the chain so the length of the tool's body contacts it, the chain is shot. The side marked "S" is for steel cogs and the "A" side is for aluminum and titanium cogs, but I just use the "A" side to get almost infinite life out of my chainrings and cogs, even titanium ones.
By replacing your chain when you should, your cogs will last at least three times as long as your chains.