The chance, etc. of a lifetime
Definition 1 of 1
Pro Tip 1/3Emotional Weight
This phrase often carries excitement and significance; it's used for opportunities that feel destiny-changing. 
Winning the scholarship was the chance of a lifetime, setting his future in motion.
Pro Tip 2/3Not Exaggeration
Use this phrase for truly exceptional situations, not common or minor opportunities. 
Meeting the renowned author for a mentorship was the chance of a lifetime, not just a regular event.
Pro Tip 3/3Serious Tone
This phrase is serious and may not suit light-hearted contexts; it implies a significant impact on life. 
Being selected as the astronaut for the Mars mission felt like the chance of a lifetime.