when i change my site design,google drop my pr to 0,because there has a lot 404 pages,my question is how long can i get back pr ? thanks
Why would they damage you for 404? If you have the option, I would do 301 redirects from the old links, and you will get back the linkjuice next time google traverse the links
I don't think they would slap you for a bunch of 404 pages - could be another reason. But then again, a bunch of broken links - maybe they would slap you for it. Don't know for sure.
Why wouldn't they? 404's are extremely bad for your PR. You should have set up a redirects on all 404 pages before even changing your design.