How to run cron more often than every hour?
You can get really fine-grained control over your cron runs with these modules:


Elyisa cron


  Elysia Cron extends Drupal standard cron, allowing a fine grain control over each task and several ways to add custom cron jobs to your site.

Ultimate Cron


  The Ultimate Cron handling for Drupal.
  Runs cron jobs individually in parallel using configurable rules, pool management and load balancing.



(In no particular order) I've only used Elysia cron before, and only recently at that, but it works well.

Either module will let you use a standard cron rule to define it's frequency, which I think means you can get the granularity down as far as seconds if you need it.

I think the only gotcha is that the more often you run cron (if it has expensive tasks to perform), the more load there will be on your server. Testing and benchmarking are the only real way to solve that one :)