Hi Bodo,

The code is pretty self-explanitory - 

"First login program assignments are hourly, but everything else is run daily - course completion etc is daily. 
You can edit the settings here: 

/totara/program/cron.php 
program_hourly_cron() { line #58 
program_daily_cron() { line #84"

Hope this is useful,

regards,

George.