How long will Preschem’s Preservatives last?

This is subject the the environment the timber is in and how much persistent moisture is present, but for most normal situations:

Timber Preserver: 4-5 years
No-Rot: 4-6 years
No-Rot Gel: 4-5 years (note that exposed exterior timber must be sealed with a quality exterior paint, otherwise the expected life will be 6-8 months)