I left it well over 12 months before I upgraded to Leopard, there seemed to be so many issues with it. I eventually upgraded when I wanted to start writing iPod/iPhone apps. It doesn't seem like a massive problem this time, but I think I'll give it a couple of months to settle/mature a bit before I do anything.

How do the upgrades apply? I upgraded to Leopard from Tiger about 3 months after I'd upgraded my HD. I'm now thinking about upgrading my HD again, if I've already upgraded to Snow Leopard, can I do a fresh install with the DVD I get as part of the upgrade? Or would I have to install Tiger from the original discs, then Leopard, then Snow Leopard?