how to delete client's mysql by scheduled time?

When are you looking to schedule the deletion? Everyday, everyhour? BTW, if you will be providing one test cPanel login, all the databases will be created as "username_dbname", this will make it difficult to track down which databases were created recently and are been testing by clients and which are not in use (ofcourse we can delete them according to their creation time) but what criteria are you looking to set for the deletion?

 

You could try using incrond to check the contents of /var/lib/mysql for modifications, and then have it take action whenever something is changed in /var/lib/mysql for that set user (it can do search patterns or exclusions).

Unlike a cron, it doesn't base these on time but on a change happening in a directory. There's a recent great tutorial on using it at NixCraft:

Linux incrond inotify: Monitor Directories For Changes And Take Action

I think it might be the better option over a cron here because any changes to /var/lib/mysql for that set cPanel user name could be checked by incrond and then those databases cleared in some manner after a time delay when the action occurs (so clearing the database an hour after a table was added to it for example). It's up to you how you want them to be cleared, I'm just pointing out a better option over a cron or script based on time.