Full tune up time! I'll be replacing my plugs at 60K ...currently just over 46K now on my GT. 100K is a bit long to go unless you drive 100% highway at speeds under 65mph. Higher rpm increases wear/heat.
In normal service (water pump aside) you shouldn't have to replace the timing chain in an engine. Knowingly, the vehicle will rust through the floor and frame before a chain replacement is necessary. I have V6/V8 fleet trucks with over 350K miles and never taken apart. Longest running engine now is the Astro 4.3L V6. My older Ford van threw its 3rd and final transmission, although the 5.4L V8 was still going strong.
Fact about my G6 GT: In 74,000kms (46K miles) on my G6, I've only replaced the engine oil 5 times (using full synthetics) - in 6 years.
'11 Ram SXT 4x4 quad cab - 5.7L V8; 92,200kms
'06 Pontiac G6 GT - 3.5L V6; 136,000kms
'12 Chrysler 200 - 3.6L V6; 79,000kms