If I buy a PR5 link for my site from another site whether my page rank will increase next moment itself or whether my page rank will be updated only at the next PR update
It will probably increase your PR but you will see changes in Toolbar PR about 3 months later or you will see none. Because with one PR5 link you can't get easily PR 6. Why would you buy PR5 link ? You can buy links for bringing some related traffic or boosting your rankings, but for increasing your PR - I think that is wasted money.
Don't write bullshit. With High PR you only get maybe some traffic and G bot more often crawls your site, nothing else. Rankings got nothing with PR. Quality topic related BL's can improve your rankings.
zokiii is right - all depends of site traffic, and traffic quality and category. pr5 just is for marketing with not educated clients