[greentriple]

Assuming on .40 G27 u should replace OEM guide rod and spring every 2000-2500 rounds.

When should u replace a steel guide rod and spring?

[JR]

There is no need to replace a steel guide rod. It will provide service forever.
LWD does not carry replacement springs for the G27. I recommend you check with Wolff Gun Springs for these items.
You would replace the spring on schedule.
If you have a Gen1,2,3 single stage recoil spring tou can see a few spring choices here http://bit.ly/ppLLXK