I'm creating a Twitter app that requires checking Twitter for mentions of my account, then processing those tweets and either reposting or redirecting them. The cron job just runs a small java jar file (200-something KB, I think), which performs all the necessary functions via the Twitter API. I'm pretty sure there's no way to have Twitter automatically process the tweets as they come in, since that would likely involve coding on the twitter server itself, not on the client end.
Anyway, updating every 5 minutes seems to be okay for now, but I just got whitelisted by Twitter (no more rate limiting) so I thought I would take advantage of my newfound freedom and have the software update more frequently.
(I should mention that the java program takes only a few seconds to run, and I don't think it eats up too much CPU power/time...)