Ok, so how many Th/s do I need before I dare to go solo mining?
30? 50?
You can Solo mine with any amount of GHs. You could do it with even 1 GHs but you will probably never hit a block. At this current network Diff, you will need around 2 PHs to hit around 1 block / day.
There are pools out here that have close to and a little more than 9 PHs and still go days without blocks. I would say that is it possible to get lucky but unreasonable.