With Intel's tick/tock strategy I would say Sandy Bridge will still be good for 3 or 4 years at least. Ivy Bridge was a tick ( or die shrink ) and is only roughly 7% faster clock for clock than Sandy Bridge. Next year Haswell will be released and should be a major architecture change ( or tock ). How much faster it wil be remains to be seen but I would expect somewhere around ~15% faster than Ivy so around 20% faster than Sandy. Then in 2014 Broadwell wil be the tick ( die shrink ) for Haswell. So by Broadwell you should see ~25% faster clock for clock speeds than a Sandy Bridge chip.
I believe Intel is going to change things up with Haswell. I think they will be more focused on reducing power consumption than increasing performance. They are looking beyond PCs and laptops for Haswell since those market segments are not growing as much as they used to. That means trying to bring out low power CPUs for the tablet market which is the new growth segment. There will be a performance increase over Ivy Bridge, but it will be at most 10%. The new "Intel HD 5000" graphic core might see 15% - 20% improvement over the HD 4000.
Broadwell will be the CPU that offers over 10% performance for the CPU and likely a 30%+ improvement for the IGP. The die shrink down to 14nm will allow Intel to produce CPUs that are more powerful than Haswell, but consume basically the same amount of power. Since tablets do not need the same performance level as desktops or laptops, Broadwell could offer a little better performance while consuming less power than the Haswell variant.