Or how long do I really have to wait to be "sure" that the generated block + 50BTC contained within it are valid?
The default on the bitcoin client is 120 confirmations.
How would somebody manage to "trick" you for 119 confirmations in this day and age at the current difficulty level?
As soon as 1 or perhaps 2 confirmations come in I fail to see how you could declare an older block invalid.
Single confirmation rewrites are not terribly uncommon. However, the odds that someone will try a double spend attack, there will happen to be a one confirmation rewrite, and one side of the rewrite will have one transaction and one the other are pretty low.
Consider if two miners each find a block at about the same time. One side of the network starts working off one block and one side starts working off the other block. Now imagine if both of these sides also find a block at about the same time, so the network splits again. Of course, the odds of this keeping up for very long are staggeringly low.