There are clear rules/guidelines. In pretty much every Country in the world entry is at the discretion of the Immigration Officer you face on entry. If you're pushing the boundaries then you may well be denied.
Visa exempt extentions are still 30 days but that will depend on how many visa exemptions you've had and as always dependent on the Immigration Officer dealing with your application.
Tourist visas are still and always have been 60 days(they are valid to use for 90 days from the date of issue), apart from the DTV which is 180 days.