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?
If I steal a hundred dollar from a loot of one thousand, people might notice;
If I steal a hundred dollar from a loot of one million, I might get away with it;
If I wish to steal even more and still go unnoticed, I need to make the loot bigger.
Now you know why taxes always go up.
Looting: «the plundering of public assets by corrupt or greedy authorities» (Wikipedia)