If I understood things correctly, not that long. Exactly as long as self destruction lasts. Re-buy screen, rebuy default ship in default port, I'm back in the bubble.
I don't think that's right. Now the C&P rules have been tweaked a few times, and I can't find a source that succinctly describes how things work as of now, so I could be wrong. But if you blow up, the two options you are presented with are A) rebuy your current ship at some location, or B)
lose that ship and spawn with a Sidey at a starter system. If you're in anything you've invested credits or engineering into, no way to take option B. For option A, my understanding is that if you exploded in an unpopulated system, you respawn at the last place you docked. Or if you are wanted there, you are shunted to the nearest detention ship.
So for the majority of gankers out at WP3 or WP4, self-destructing only gets them back to Omega or Rohini or somewhere similar. Heck, if they both docked and did crimes at Sacagawea, self-destructing would actually land them *farther* from the Bubble, since the nearest detention center would be further down the Colonia highway.
For what it's worth, if you really wanted to preserve the ability to warp back to the bubble, I think you would do the following: A) make sure you last docked at a base with a shipyard (so Omega or Rohini so far), B) self-destruct and take the rebuy to that base, and then C) suicidewinder (i.e. buy a Sidey, explode, and don't rebuy). Note that this leaves their gankship stuck at the remote shipyard, but the people in question are mostly billionaires so transferring the ship back or leaving it there for later probably isn't a hardship.