you can activate the regeneration ability whenever you have priority ("instant timing")the last opportunity is before the creature is actually destroyed, so for example in response to Doom Blade or in the Declare Blockers step (just before Combat Damage)
The Basic rulebook, read it! A lot of basic questions are answered there!
How to autocard :
Type [c]Black Lotus[/c] to get Black Lotus.
Type [c=Black Lotus]The Overpowered One[/c] to get The Overpowered One.
When do I pay the regeneration cost?
Insanity: doing the same thing over and over again and expecting different results. -Albert Einstein
So, you keep on voting, and you keep on expecting different results from those elections?
How long have you been doing this, over and over again?
Ain't it time you reached a different conclusion about this whole masquerade?