Once In A Lifetime Opportunities...

...show up every day.  The key is knowing when to say yes, but more importantly, when to say no.  Try running the opportunity through Jim Collins' Hedgehog Concept:

1.  What can you be the best at?
2.  Does it make a profit?
3.  What are you really passionate about?

If the opportunity makes sense after considering those three questions, then it might make sense to pursue it.