I usually go every Oct/Nov, late I guess, but it's been working as my vehicles are all rust free, despite some seeing many years of heavy Ontario winter use. I used to go again in the spring for a second application, but now I care less, so once a year it is.
I can't understand why anyone who owns their vehicle and intends to keep it for at least 5+ years wouldn't get it sprayed. It only takes one good look at an untreated car's underside after a couple of years to see why. The 2012 Wrangler with 100,000kms I have for work is rusty in places I was surprised to find rust in that early...inner fenders, body mounts, etc. Any in some places beyond just surface rust and getting pretty flaky.
My 96 Firebird on the other had, has seen 20 years of Ontario winters, sprayed every year, and the underside is mint...not a trace of rust, except for some surface rust on some el cheapo (unpainted) rear springs I put in years ago.
