When is the best time to build new high end PC?

Well the biggest thing will probably be the new gpu's they will probably be the biggest change. I can only guess that intel will come out with a new socket but 2011 socket will be like what the 1155 socket is now. And as for motherboards sure new ones will come out but I can't think of any major differences that could possibly take place. So it really depends on what your budget is and if you will be able to afford these thing.

As for a time frame my best guess would be at the end of the first quarter.

the best time is black friday/cyber monday. tons of sales. if you wait the for the newest stuff expect to pay top dollar. just go look what amd wanted for the 7000 series when it first released. haswell is coming in june.

I can afford up to 3K USD that's why I feel we are close to end of the 7000 Gen and new gen will release but any idea when ?

imo, we will see (expected) hd8000 gpus during Q2 2013 and new intel cpus in H2 2013

so Q3 2013 seems a good quarter to watch for bew stuff and bench leaks

and , if new techs dissapoints at that time then you can go fo i7-3930x + asus rampage + 2x hd7990 crossfire as they may see price drop to clear stock at that time