From NexusWiki

The Nexus unit of level advancement. At level 99, experience is used to buy vita and mana, as well as Might, Will, and Grace.

The amount of Experience required to advance to the next Insight increases progressively until the character is level 99.

Currently Nexus has experience capped at roughly 4.29 billion. Which simply means, that's as much as you can carry at any given time. Once you sell your experience for vita, mana or stats then you will be able to acquire more experience.