Two 3080s = $1500 + number of hours times electricity cost (for me, this would be about $0.14/hour)

A K80 from AWS (close enough to the same RAM) = $0.45/hour (spot pricing) or $0.9 (on demand)

I've been able to use spot instances for basically all of my AWS hobby work, so I'll use that.

The crossover point where it's worth buying is where 1500 + 0.14h = 0.45h, about 5000 hours or 30 weeks of training.

For hobby work, do you expect to have it training for 20% of the next 3 years? That's all without considering the fact that you might want to upgrade to a newer card, AWS's prices will likely fall, and you might want a card with more RAM sometimes, but not others, or even a multi-GPU setup sometimes.

I used to spend money on fancy cards and machines and justify it with my hobby learning. Now I just let somebody else do the heavy lifting and pay them rent. It's spoiled me. You go from spending a few nights trying to get a thing performant to throwing a burrito worth of money at amazon to just parallelize the dumb thing on a massive machine and having it done by bed time.

I also used to justify buying the fancy card with my gaming hobby, but then when I actually did have a long running training job, it'd be super frustrating because now I can't use the desktop for anything else for a few days until training finishes.