life‧time [countable usually singular]
the period of time during which someone is alive or something exists [↪ lifespan]
during/in somebody's lifetime
During her lifetime she had witnessed two world wars.
It's the sort of opportunity you see only once in a lifetime.
the best opportunity, experience etc that you will ever have:
It was the holiday of a lifetime.
not at all [= never]:
'Would you go out with him after he dropped you?' 'Not in this lifetime.'