In SharePoint 2010 if you add a keyword and attach a Best Bet to it, it is usually made available right away. But if your system has a lot to do, then the timer jobs kicking this into production might not run as often as you desire.
This is easily fixed by running the “Prepare query suggestions” timer job. Seems this jobs handles anything to do with synonyms, nicknames and also keywords.
In a SharePoint Management PowerShell window you can start the timer job by issuing
Start-SPTimerJob -Identity "Prepare query suggestions"