No one knows for sure.
we are all speculating based on whats happened in the past, but based on the releases lately, pushing iPhone back, iPad 2. theres really no telling. But im willing to bet that since this release was such a leap forward, and it just happened, and they pushed back iPhone 5, that there will not be another release before August. Or even this year for that matter.
Just buy now, and besides even if there is another update, are you really going to need the stuff included in the update? i mean a 2.2 quadcore CPU with a SSD and a HD screen is more than enough for the avg user and is enough for most advanced users!
EDIT: Since they are about to refresh the iMac, and possibly the MacPro, i doubt they will honestly have time to form a revision of the 2011 MBP with all of these new things being produced. i mean think about it, around august they will have in production (or very close to it) an iPhone 5, iMac, MacPro, ( possibly, very unlikely but maybe) iPad?