I haven't had a problem with firearms sitting a year or more dirty as long as they are in a spot where the oil can't collect dust.
Even when I do a deep cleaning now, it's just Seafoam Deep Creep to soak it all down then paper towels to pull off the big dirt followed by a microfiber gun cloth to get the rest. Maybe a couple of passes of a bore snake if I'm worried about fouling but I just don't see any difference on the borescope so why add the work and wear for such little result.
These needle oilers are the greatest thing: Amazon.com: TITGGI 5 Pcs Needle Oiler (1 OZ), Precision Gun Oil Bottle, LDPE Oil Applicator with Extra Long 1.5 Inch Stainless Needle Tip – Easy to Use for Gun Oil, Crafts, Hinges, Sewing Machines and More : Industrial & Scientific. I fill mine with this: Amazon.com : Hoppe's Elite Gun Oil, 4 oz. Bottle : Hunting Cleaning And Maintenance Products : Sports & Outdoors. Oil up the gun before you shoot following the directions that came with it, and you'll never have a problem. For those classes where you shoot 1000+ rounds in 2 days you'll probably want to oil after lunch too.