[ASUS] Z77 Sabertooth - How long will this last for..?

It's up to you how long you'll be willing to have it last. It should be able to run the highest settings for at least the next two years at least but then you'll possibly want to consider upgrading the GPU. As for the motherboard it's fine but it's made for durability more so than performance (the best models have more than 2 PCI-E x16 slots). I'd just go for a 7970 personally and overclock it saving about $80-100 over the 680 and have the same performance. The PSU you listed is complete overkill unless you want to run three graphics cards.

So to sum it up that'll last a long time but going on past a few years you'll have to lower setting to keep games playable (maybe medium since you're looking at top of the line at the moment or high instead of ultra).

I am not planning to upgrade the GPU. I will just get another GTX 680 4GB to SLI.

It's good then if you're stuck with the 680 but I'd still recommend a 7970. Also by upgrade I normally mean either add a second (much better for the wallet and performance).