the fastest will be on FF and -40C and slowest on SS 175C on TSMC FAB. ring oscillator has quite a big variation, only use it on blocks that are not timing sensitive...
If you need a ring oscillator for PLL kind of application go for the fully differential ring oscillator .. they have an innet property of robustness against porcess and supply variations.
For ring oscillator itself, the output frequency will vary from -40% to +40%, PVT. To assure 30MHz output frequency, you either need calibration-trimming or PLL to lock the frequency
As all the previous guys already said, the frequency variation is considerable. It may be even bigger if you use lower supply voltage. The main reason behind it is the delay of a basic cell. This delay is a relatively complex function of different parameters (although, there are simplified models for its calculation), and is very sensitive to process variations.
yes you need to use trimming to trim the frequency across corners. But variation across V-T can also be high enough, but atleast you can remove the variation across process by having sufficient trimming.